The standard hand guards snap into a retainer at the front, behind the rear sight base, and at the rear to corresponding recesses in the "delta ring" (the round spring tensioned piece you have to pull rearward to get the hand guard to disengage). This means the hand guard is supported partially in the front by the barrel. Any load you put on the hand guard is transferred to the barrel which affects accuracy.
A free floated hand guard usually has a replacement barrel nut with threads on the front as well, and if it has rails or provisions for rails, will have an indexing ring for proper alignment. The actual hand guard, or "tube" then threads onto the barrel nut and is entirely supported by the upper receiver via the barrel nut. it doesn't (shouldn't) touch the barrel at all therefore pressure on the hand guard never transfers to the barrel.
Secondarily, they aid in heat dissipation if F/A or rapid fire is your thing. Some are vented to further help disperse heat. Even if they're not, no contact with the barrel means no heat conducted to the hand guard surface.
Some manufacturers offer factory free float hand guards. If yours is bone stock base model and slips into the retainer behind the front sight post it's not a free float. Or, if it's plastic, chances are pretty good it's "standard". Free float "tubes" are just that. Usually a one piece cylindrical tube (some have various recesses cut or machined in). JP does offers a trapezoidal