Zaps are micropayments to other nostr users. Zapping is the action of sending sats to someone.

Sats are an abbreviation for Satoshi, which is the smallest fraction of bitcoin. (1/100M of btc)

So zapping someone is just sending him some money.

In order to do this you need to get a lightning network (LN) wallet. You can get one very easily by downloading Wallet of Satoshi from your app store. Then you just copy paste adress of that wallet into whatever client you are using. (usually you can find this easily somewhere in settings)

As for reasons why you would do that, there are many and none. You may send a couple sats just to show a more tangible appreciation of a post than a simple like or zhank you comment. You mqy send some just for shits n giggles. You may do so, to help a fellow pleb in need, or for literally any reason you can think of.

For instance #zapathon is quite fun and will be taking place later today. Starting at 22GMT0 and then for the next 30 minutes we all zap whatever notes include the #zapathon tag. It's a cultural thing, which you may end up loving or ignoring completely.

Zaps are not strictly neccessary for using Nostr, but they provide an enhanced experience. Receiving even a couple sats, is far more appealing than receiving likes, because it means, what you post is actually worth for others enough to send you some money.

The beauty of Zaps is that, you can make someone smile and feel appreciated for literally a cent or two. That's very cheap happiness and it is very addictive.