Explainer video might help - https://twitter.com/thecryptoc0up1e/status/1622380473849892864?s=46&t=hYH_MC1HlvsF4WvTi58loQ

Paid relays are basically lines in to nostr that you pay for. Spammers don’t pay so they use different lines and on nostr we have to be using some of the same lines to see each others notes so you will eliminate spam and support the network by paying the relay operators. Full list on https://relay.exchange