Free float is referring to the way the rail attaches and interacts with the upper receiver and barrel. When you fire a bullet through a barrel, the barrel vibrates and moves in very specific ways. If the handguard or rail is physically touching the barrel, any interaction you have with that handguard will negatively affect the rifle’s accuracy. With a free float rail, the barrel is completely free to vibrate and move consistently, which drastically improves repeatable accuracy.