like another poster said
it would be 2 players chatting outside of the poker room chat and formulating strategy
they would at a minimum tell each other their
hands ... they could plan to steal blinds by deciding to have one raise and the other re-raise to show strength and then they might just check it down after that with marginal hands
one player could "soft off" their chips to the other
it's certainly not fool proof and it can be easily detected and reported ... the poker room would/should just forbid those players from playing at the same table even with little proof, since it does not prevent either player from playing
in a similar way as if you transfer money to someone, then u can't play at same table
something to watch out for, but i wouldnt fear playing online b/c of collusion ... good players dont want to mess with cheating ... they feel their skills give them all the edge they need and while 1 or 2 reports against you might not prevent you from playing ... get 3+ and i imagine the poker room would ban you from
real money games