It connects to relays like a client and stores the notes. It acts as a relay you connect to. That way you get the notes from all the relays while only connecting to one. It’s more efficient.

GitHub

GitHub - PrimalHQ/primal-server: Primal Server includes membership, discovery and media caching services for Nostr

Primal Server includes membership, discovery and media caching services for Nostr - PrimalHQ/primal-server