Assembly question

I starting to learning assembly,and really insterested to read "The art of assembly language Programming" but stuck in chapter 1.6 "Logical Operations on Binary Numbers and Bit Strings"

Here is the text from the book:

This bitwise logical AND operation would force the H.O. four bits to zero and pass the L.O. four bits of 'X' through unchanged. Likewise you could force the L.O. bit of 'X' to one and invert bit number two of 'X' by logically ORing 'X' with 0000 0001 and logically exclusive-ORing 'X' with 0000 0100 respectively. Using the logical AND OR and XOR operations to manipulate bit strings in this fashion is know as masking bit strings. We use the term masking because we can use certain values (one for AND zero for OR/XOR) to 'mask out' certain bits from the operation when forcing bits to zero one or their inverse.

Online book was here:

http://oopweb.com/Assembly/Documents/ArtOfAssembly/VolumeFrames.html

The question is:
- what the hell is "0ring???
- and i am a bit miss understand in this text.would someone explain me by an example operation?