Zaps are Satoshi sent to each other via a Bitcoin Lightning wallet.

Once you get a Lightning wallet set up, you can paste your lightning URL into your Nostr profile and start sending and receiving zaps.

Zeus and Phoenix are self-custodial wallets that support Lightning. These take around $100-$120 to open a lightning channel with the minimum 100,000 Satoshi in them.

Minibits is a non-custodial wallet, but you can get it working without putting any money into it.