I find a different definition of metapuzzle

Meta puzzle definition from wolfram alfa and youtube

So what meta-puzzle actually is ?

Those refer to "meta-logic puzzles", which we call and .

A in the puzzle community is a puzzle that combines answers from other puzzles to get a final solution. For example, you might have this list of answers from solving seven puzzles:

ARRANGE
CANCELLING
CROOKED
SCRABBLE
SEVENTEEN
TTYL
VACUUM

You could notice that

each of the answers has a double letter. Also, the double letters are all at unique positions in the word - TTYL has it at the beginning, ARRANGE has it after one letter, CROOKED has it after two...

You would then sort them by how many letters come before the double letter:
-TTYL
-ARRANGE
-CROOKED
-VACUUM
-SCRABBLE
-CANCELLING
-SEVENTEEN
The double letters now spell out TROUBLE, and that would be the answer to the metapuzzle.

Metapuzzles can be a lot more complicated than that, and sometimes they use something else too (like a diagram that you have to fill the puzzle answers into). This is the basic idea though - a puzzle made out of answers to other puzzles, where you have to figure out how to combine them.