1. Definitions and callouts
Holes
Definitions and callouts
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2. counterbore
A counterbore can refer to a cylindrical flat-bottomed hole, which enlarges another hole. A spot face is a very shallow counterbore.
This the callout for a counterbore hole. It gives the diameter of the top and bottom holes and tells how deep both of them are (the top, larger hole is .266 deep, the bottom, smaller hole goes all the way through)
Wikipedia.org

3. countersink
A countersink is a conical hole Usually used to allow the head of a countersunk bolt or screw, when placed in the hole, to sit flush with or below the surface of the surrounding material.
This is the callout for a countersunk hole that goes all the way through with a 2.06 diameter and has a top diameter of 3.71 that angles down at a 100 degree angle

4. Tapped holes and clearance
A hole with threads is called a threaded hole or a tapped hole.
To tap means to cut threads into a hole.
A clearance hole is big enough for a threaded screw to pass through. The threaded parts need to be the same size.
The callout for this tapped hole tells that there are 4 identical holes (4X) the type of thread (3/4-10 UNC), and how deep it is
The clearance can be close, loose or normal

5. Thru and blind holes
Where it is not clear that a hole goes through an object, the abbreviation “THRU” follows the dimension.
A blind hole does not go all the way through.
Here is a blind hole that is only .750 deep