Cobblemon servers are based on the popular Minecraft mod Cobblemon which adds Pokémon into the world and lets players catch Pokémon, build a team, and battle other players. Many Cobblemon servers add extra features like gyms, breeding, NPCs, tournaments, and mounting which lets you ride around the map on your Pokémon. Griefing is usually not allowed at all so servers often have rules against it and use land claiming to help protect player builds. Regular PvP is usually less important as well since most of the gameplay is based around battling, collecting, and training Pokémon.
Before joining check which modpack it requires since the Minecraft version and mod loader need to match. Most servers will list whether they use Fabric, NeoForge, or a custom pack with extra mods in the description.