In the episode this scene is from, Spongebob goes around and enters other people's dreams while they sleep. While everyone has dreams about their desires (Plankton wants to be all powerful, Krabs desires money, etc), Patrick's is literally just him riding the seahorse the entire time. Spongebob tells him he can do anything he wants in a dream to which Patrick just responds with "Cool", before Sponge leaves out of boredom.
While everyone else wishes they could achieve more than what they currently have, Patrick is content to just live a simple life of enjoying the little things. Growing up, we're told that we can be whatever we want to be - a football player, a spaceman, a king, etc and that we're lazy if we fail to live up to these false expectations of us. The blunt truth is, 99% of us will ultimately amount to nothing, and there's absolutely nothing wrong with that. Life is meant to be enjoyed however you want. So what if society tells us we're losers for not climbing up the ladder? It's more fun and less stressful to just not climb it sometimes.
Patrick on a seahorse represents what we all want: a content, simple life without the pressures that society keeps putting on us. They laugh at him because he sits under a rock all day, but he's happy to do it, and I think we can all strive to be more like Patrick in our lives.
Odell Lake Is The Best Game Ever Made