In the book Practical English Usage, on page number 253:
"Because no matter who, what, etc act like conjunctions, they must be used with
two clauses.
- No matter when you come, you’ll be welcome, (but not No matter when you come.)
to introduce just one clause, we can use It doesn't matter.
- It doesn’t matter when you come."
I haven't understood what this quoted text means. can somebody tell me what it means?