A zap is a direct payment. The numbers you see are actually 'sats' or 'satoshis' the smallest unit of a bitcoin (0.00000001 ₿). With the current exchange rate of about 100k$/BTC the math is actually fairly easy, roughly 1000 sats per dollar.

This means Ian right now roughly made 670$ worth in bitcoin on his post right now.

You will have to set up a wallet and connecy it to your Nostr profile in order to receive zaps. There should be enough documentation out there, and otherwise ask around and hopefully someone will help you on your way.

Welcome and goodluck :)