(a) "Let him who believes in Allah and the Last Day speak good, or keep silent; and let him who believes in Allah and the Last Day be generous to his neighbour; and let him who believes in Allah and the Last Day be generous to his guest."
(b) Whoever believes in Allah and the Last Day…”
In this hadith, Prophet mentions 3 features that need to be a part of a Muslim’s character in order for him to complete his/her faith/imaan. Therefore, if you find yourself lacking in these qualities, this hadith should serve as a reminder and a reality check, helping you reassess and refine your goals to be the best version of yourself.
So, why it the Last Day mentioned?
This serves as an encouragement and pushes you to strive to be the best Muslim you can be, because every single aspect of your life will be questioned on the Day of Judgment, including how you treated others and how you carried yourself in this dunya.
“…should speak good things or keep silent…”
The first part is concerned with how we use our tongue. Needless to say, majority of problems we face is due to incorrect usage of tongue, a part of your body that needs to be guarded at all times. It’s no secret that excessive speech will always lead to problems and misunderstandings that would have never crept up in the first place, if only we thought twice before opening our mouths. Our speech is one of the quickest ways for us to collect unwanted sins or blessings depending on how we choose to use it. So next time, you’re about to talk, ask yourself:

Will my speech by pleasing to Allah?
Is it absolutely necessary to talk?
Will I be overstepping my boundaries if I say such-and-such thing?
Will this speech cause happiness and bring about good will or will it cause souring of relationships and create bad blood?

Of course, you need not limit yourself to these set of questions. There are many more aspects which you can assess for yourself before talking.
Umar r.a. once said,
“The one who speaks a lot commits lots of mistakes. The one who makes lots of mistakes commits lots of sins. The one who commits lots of sins will have the Hell-fire as the first place for him.”(Ibn Rajab)
Therefore, use your speech to:

Correct people when they commit mistakes or are straying away from the path of Allah
Prevent people from committing sins
Set relationships right for the greater good
Guide others towards righteousness

And remain silent so that you…

Don’t spread false information about others
Don’t lie, it may be tempting but the side effects are much worse than what you could have imagined.
Don’t backbite: Refrain; refrain from it as much as you can!
Don’t speak about this deen when you don’t have sufficient knowledge to do so

“…should be courteous and generous to his neighbor…”
With advancement in technology, we have become obsessed with our gadgets, smartphones and laptops, so much so that we don’t spend enough time with our own family members, let alone visiting and checking out on our neighbors. This hadith is a timely reminder sent by our beloved Prophet himself, insisting on how our conduct should be with our neighbors, irrespective of whether they are Muslims or not:
What you can do:

Smile: Yes, smile at your neighbors and those around you, not only is it sunnah, but smiling is the fastest way to cement and bond and relationship that can last a lifetime. Initiate this beautiful sunnah of our Prophet.
Ask if they need any kind of assistance.
Visit them when they are ill.
Prepare nice food for them.
Be the kind of person whom they are delighted to meet and greet when they see you.

Abu Bakr al-Jazairi (reference: Minhaaj al-Muslim) goes on to add:

Congratulate them if something pleasing occurs to them,
Offer condolences upon afflictions,
Help them if they are in need,
Be the first to greet them,
Be kind in speech to them,
Be gentle in one’s speech to the neighbor’s children,
Guide them to what is best for their religion and worldly life,
Overlook their mistakes,
Don’t attempt to look into their private matters,
Don’t constraint them with your building or renovations or along the walkway,
Don’t harm them by letting one’s trash onto their property or in front of their household.

The Prophet said: “Jibreel (as) did not cease to advise me regarding the rights of the neighbor, until I began to think that they would also receive a part of the inheritance”. [Al-Bukhari and Muslim].
These small steps can bring tremendous rewards from Allah, so start practicing them now!
“…should be courteous and generous to his [traveling] visitor”
Here, we are being commanded to be courteous and generous to a traveling person who has left his belongings/family/property in search of whatever he/she is traveling for. So how does a Muslim treat a traveler who is in a foreign land and could be extremely apprehensive because he knows no one?

By being kind towards him
Ask what his needs are and try your best to fulfill them
Prepare the best of food and give it to them, without extravagance

The scholars say “Belief is what settles in the heart and manifests itself on one’s limbs” therefore, if you truly have faith in your heart, these actions and a whole lot of righteous deeds will automatically flow through your limbs and tongue! And if they don’t, don’t worry, ask Allah to make you a beautiful person inside out!