Levels of retention
The retention level of the holster refers to the number of retention devices you have to release or move the gun past in order to draw the pistol from the holster:
An example of a level l holster would be a simple thumb-break device that must be unclipped in order to draw the weapon.
A Level ll holster example is one where a thumb-break is released then the pistol must be moved (rocked forward or rearwards) in the holster to clear some form of internal locking device before it can be drawn.
A Level lll holster is one where three separate retaining devices, both internal and external must be undone or bypassed.
Much more information at the linked page.