Simply put, a Blunder is the term given over to really bad move.
Blunders can be caused by a player being too cocky about their ability - overly confident, they'll play a move, almost on autopilot, without fully considering the situation and only when it's too late will they realize their error - usually when a key Piece is captured, or their opponent nicks in to grab a superior position, on the Board.
Blunders are usually mistakes made by experienced players who, on other occasions, would see the error and simply not play into it ...
You could argue that Beginners don't make Blunders, because they don't know they're making mistakes through lack of experience - but, let's skip that argument, as we'll be here all day.
Typical circumstances for Blunders to happen include:
Guarding Piece goes AWOL. Not being aware that the Piece you're about to move is guarding another important Piece ... You go move that guard and that other Piece subsequently gets captured.
For instance, you move a Knight that was protecting your, otherwise unguarded, Queen and then your Queen gets captured (and it all could've been avoided!).
Promotion to a Queen gifts Stalemate to your opponent. Sometimes, a player can focus so much on getting a Pawn up the other end of the Board, to Promote it to a Queen, that, by the time they reach that goal, they don't stop to assess the situation ...
They're on the way to victory; they have the greater force and the advantage appears to be on their side ... Triumphantly, they slap a Queen on the Board; then spontaneously combust, as they realize their opponent's King was in a Stalemate situation, which has just been triggered by the Queen's entry.
If you get to Promote a Pawn, STOP and assess the situation, before deciding which Piece to get - it may be wiser to Underpromote your Pawn to either a Knight, Bishop, or Rook.
Allowing a Piece to become Trapped The most dire outcome, here, is when it's your King and the result is loss by what's known as Smothered Mate.
However, other Pieces can become Trapped - especially those that cannot jump to escape (which is all, except the Knight) ... All it takes is careless position of Pawns and a few other Pieces and, all of a sudden, you may find your Bishop/Rook/Queen unable to escape from, say a Pawn, or Knight.
Besides those examples, you also need to be aware of the patterns that makes it possible for certain Chess Tactics to be implemented ...
Failing to spot a potential Pin Attack pattern. If you have a less-valuable Piece, or a Pawn, in front of a more-valuable Piece, this pattern is ideal for your opponent to exploit, with a Pin.
Failing to spot a potential Fork Attack pattern. Having two, or more, valuable Pieces on the same colored diagonal; or on the same Rank or File, could lead to your opponent catching you with a Fork Attack - you'll be forced to choose to which to save and which to lose ...
If your King is one of the attacked, you will HAVE to rescue the King at the expense of the other - even if your Queen is the victim.
Failing to spot a potential Skewer Attack pattern. The pattern of a Skewer is the reverse of a Pin ... If you happen to have a more-valuable Piece in front of a less valuable Piece, or a Pawn in a good position; you will be forced to rescue the more valuable Piece ...
It may be less valuable, relatively speaking, but losing any material, when you've worked to get them into key positions, is a metaphorical rake down the shins.
The results of being a victim, of these Tactics, could be:
A loss of material;
To allow your opponent to gain Positional advantage (e.g. increased territorial Control);