It's like ''cool story bro''.
It's when whatever you said doesn't interest someone at all.
Example.
Someone posts a thread called ''It's my birthday today!'' in general discussion.
Naturally, I couldn't care less about your birthday, anniversary, depression or anything about you at all, so just to increase my post count I'll post ''Cool beans''.
I also use cool beans whenever I'm chatting, it's not something that's only used on forums.