什么是掩码，掩码的用途是什么

百度收到的都不精确，看我收录老外的一段话，这才准确：

A mask defines which bits you want to keep, and which bits you want to clear.

Masking is the act of applying a mask to a value. This is accomplished by doing:

1、Bitwise ANDing in order to extract a subset of the bits in the value
2、Bitwise ORing in order to set a subset of the bits in the value
3、Bitwise XORing in order to toggle a subset of the bits in the value