It's not a difficult concept to understand. I guess you are purposely choosing to be ignorant of the point.
It is important here to separate the facts from the politics.
On a factual point, A50 cannot be extended by one side without the agreement of the other. Both sides have to agree, else it doesn't change.
On a political point, the EU offering an extension puts the ball back in the court of UKG to either take it and negotiate, or reject it and shoulder the blame for No Deal.