A zap is the same as saying a "tip" or micro payment. The currency behind them is bitcoin. The bitcoin network is not great for small instant payments, so we use the Lightning Network which is a layer 2 solution to make payments more flexible and instant. Lightning is where the ZAP term comes from.

Each bitcoin is made up of 100,000,000 SATs (short for Satoshi). You can zap someone 21 SATs for instance. If you see something you really value, you can zap a lot more. I'd recommend keeping the zaps under $10 or so until you get the hang of things.

You can definitely set a default amount of SATs (no premium needed) and some clients have little pop ups that let you pick between a set of values.

Have fun!