Text masks are a type of vector mask. You apply text masks just as you apply masks using existing objects: you simply use text as the mask object. The usual way to apply a text mask is to use its path outline, but you can apply a text mask using its grayscale appearance as well.

This is a picture of the feature being described.

A text mask applied using its path outline

For more information, see Creating a mask from an existing object. For more information about the different ways masks can be applied, see Changing the way masks are applied.