A holster is the retention device that carries the pistol on your belt. It holds the gun muzzle-down and completely covers the trigger, so nothing can touch the trigger while the pistol is seated. It rides on your strong-hand side, with the heel of the grip above the top edge of the belt, and it keeps the muzzle pointing down - within about a meter of your feet when you stand relaxed. In practical shooting the pistol is carried in it with no magazine and the hammer or striker down.
At a match the holster is the piece safety rides on, so a few things are non-negotiable. It must block all access to the trigger while holstered, and drawing must never require the trigger finger to release the gun. If it has a retention strap or flap, that is closed before the "Standby" command. You run only one holster on the start line, and shoulder holsters or "tie-down" rigs are not allowed.
Where the holster sits is measured two ways. In every division the pistol can't stand more than 50 mm off the belt, measured to the center of the grip. And in Production, Production Optics, Standard and Classic the holster sits behind the line of the hip bone - Open and Revolver may carry farther forward. Set once and left alone, a good holster gives you the same draw every time, which is exactly what you drill at home with dry fire.