Metric thread clearance
A clearance drill bit will drill a hole for the screw to pass through such as a scope base.
A tap drill bit will drill a hole that you can then tap with the appropriate tap.
What is Tapping?
Drilling and tapping are two distinct actions.
Drilling refers to creating a smooth hole in a material with a drill and motor.
Tapping is the action that creates a thread into the side of the hole.