what is a blunder


Blunder is generally reserved for very bad moves. Usually, that means so bad as to affect the expected outcome of the game. That is,
- if you are winning, a blunder leaves you drawish or losing.
- If you are drawish, a blunder leaves you losing.
I suppose you could also call it a blunder if you have an easy win, and you make a bad move that results in a very difficult win.
These sites may help- https://www.chess.com/openings, https://www.chess.com/lessons , https://www.chess.com/videos
The analysis engine uses its numerical evaluation of the position to categorise a move as a blunder. The threshold is 2 to 3 pts. So if you were winning by 8 pts and made a poor move that reduced your lead to 5 pts, that would be a blunder. It's just a large change in evaluation that affects the game though it wont necessarily affect the outcome.
the site's analysis mouse over defines it as "a very bad move that loses material or the game"
Another way to say it: its the kind of move you make after you study the board for a long time and then intentionally pick the worst move you could find, hoping to confusing your opponent with your 'audacity'.