What's the exact role of “significant characters” in C (variables)?

Question

What is the exact role of \"significant characters\" in C , especially in the field of variables? I have read the topic “(K&R) At Least the first 31 characters...”, but

Answer 1

This is what I think,

It means that all the characters, after first 31 characters will be ignored, i.e. variable names:

ab..(27 characters)..yz123
ab..(27 characters)..yz578

will be treated as:

ab..(27 characters)..yz

and so you can get redeclaration error...