When a rifle cartridge fires, it causes the barrel to vibrate.
Anything which interferes with this vibration, like contact with a tightly-fitted fore end, can have a detrimental effect on accuracy.
In a rifle with a fully floating barrel, there is a tiny air gap between the barrel and the fore end, only the receiver being firmly attached to the stock.
Thus, when shooting, the barrel is totally free to vibrate while the bullet is still passing through the bore.
A quick check to see if a barrel really is fully floating is to loosely wrap a sheet of ordinary writing paper around it ahead of the fore-end, and see if you can slide it back along the barrel all the way to its junction with the receiver.
If you can’t, then the barrel isn’t fully floating.
To get the best out of a fully floating barrel (or any other rifle barrel, for that matter), ammunition must be spot-on consistent from shot to shot.
It is also possible to ‘tune’ a cartridge to a specific barrel by experimenting with different bullet weights and profiles, and with the volume and type of the propellant powder.
This is a very brief explanation of what goes on. Whole books have been written on the subject!