A drill chuck is designed to hold the drill bit in place while it is spinning. It usually consists of a three-way vise and a tightening collar that adjusts the vise. Some drill chucks, especially those found on lower-torque battery-powered drills, are designed to be hand-tightened. Others are designed to be tightened with a chuck key or chuck wrench. The key fits into a hole on the chuck and has a set of teeth that interlock with teeth on the collar in order to tighten it.
Adjust the vise by rotating the collar until it is open slightly more than size of the shaft of the bit you are using. If it is opened too wide it may not tighten properly or the bit may become misaligned.
Hold the bit in place and turn the collar clockwise until the vise clamps the shaft of the bit.
Grasp the collar firmly and continue to turn clockwise until it will no longer turn under heavy pressure. If your drill requires a key to tighten it, insert the key and turn clockwise until it will no longer move, even under firm pressure.