goal in puzzles

The goal is to find the best move. Not always checkmate. Not even always a winning move, sometimes you have to fight for a draw.
On another note, I looked at your puzzles. You should play slower and try to calculate stuff. Thinking for only 12 seconds and then making the wrong move is just weird.

Hi cerebov. Thank you for the explanation. l am a beginner and when you start a puzzle it doesn't tell you what's the goal.
The goal is normally winning material or checkmate. Moves are normally forced. Suggest tutorial on forced moves if you are not familiar with the concept. Though it depends somewhat on the puzzle rating.

What are the forced moves? Where can I find that tutorial? Are there tutorials about this application?

What are the forced moves? Where can I find that tutorial? Are there tutorials about this application?
Forced moves are ones that leave your opponent to realistic alternative option, e.g. to prevent checkmate or a serious material loss. Puzzle solutions usually consist of a series of forcing moves.
Here's a 14-minute introductory video: https://www.youtube.com/watch?v=XgnLqW8AltI

Great video. Thank you for sharing. One more question. When I open new puzzle there is a number that is increasing every time I finish one puzzle and start a new one. It's 454 now. What does it mean? Does it mean how complicated that puzzle is?
There is also a chess.com lesson on forced moves in the lessons section, or at least there used to be. The number is the puzzle rating, so how hard the puzzle is. If you do the Rated Puzzles, your stats will track it. If you get Rated Puzzles correct first time, it will go up, get them wrong and it will go down. You can use it to see that you are improving. For Rated Puzzles the engine will present puzzles a few hundred points either side of your rating.