Free slots generally refer to ‘play money’ slots that you can access at most online casinos whenever you like as they are free to play and don’t pay out winnings. In most cases you don’t need to open a casino account to access them.
In other words free slots are ‘demo’ versions of a casino’s real money games and are identical except you can’t cash out a cent of winnings from them.
They’re purely for fun and a great way to learn a particular slot, like how to play it, its features and its pay table before you switch to its real money counterpart.