Making mistakes and blunders is a very common phenomenon. Probably every single chess game ever played contains at least one error. There are different kinds of mistakes in a chess game. One type is called "positional mistakes", another type is called "tactical mistakes". Yet another type is called "blunders". A blunder is the extreme case of tactical errors, where a player misses one or two move combinations that cause them to lose material or become check mated.
A positional mistake is a mistake that makes a player's position worse. A common scenario is that a pawn is pushed forward and the square it used to protect becomes weak. For example, after c7-c5, the c-pawn can no longer go to c6 and therefore it cannot protect the d5-square. Another example, after 0-0 and h2-h3, the h-pawn has moved forward and made the king weaker. Black can follow up with g7-g5-g4xh3 and open the g-file to attack white's king. This does not mean that c7-c5 and h2-h3 are always positional errors. But they could be errors in some positions. Think hard before pushing a pawn forward. If you just want to make a waiting move, then use a piece instead.
A positional mistake could be to exchange the wrong pieces. Which pieces to exchange and why is a key topic in all chess games. For example, if you place lots of your pawns on light squares and then exchange away your dark squared bishop, then you become weak on the dark squares. That would be a bad positional decision. A positional mistake causes problems in the long term, its minuses become apparent only after ten, twenty or even more moves forward in the game.
A tactical mistake is to calculate a long variation and then realizing that you missed something too late, because the variation is more or less forced and you can no longer back out. Sacrificing material without enough compensation is a good example. For example, white can attack black's king with Bxh7+, sacrificing the bishop for a pawn and for opening black's king. The idea is Bxh7+, Kxh7, Nf3-g5+, Kg8, Qd1-h5 and mate on h7. But sometimes black can get away with the king and then win the game thanks to their material advantage. After Ng5+, black also has Kg6 and Kh6, both of which need to be properly calculated before playing Bxh7+.
A tactical mistake has minuses that become directly visible. A serious tactical mistake can make you resign the game after the combination is completed on the board. A tactical mistake can be explained by the fact that a player missed something important while calculating the variation. Or perhaps the player did not calculate the move at all. Assuming that the player calculated the variation before making the first move of the combination, something was missed. For example, when you threaten a piece, you expect that piece to move away. But the opponent can also make a counter-threat of one of your pieces of equivalent or higher material. Another thing that can be missed is the correct evaluation of the position that arises after the combination has finished. Evaluation of the position that is the result of a combination is an important part of the calculation process. That's all for this time.