I just don't understand

The king can't take any piece which is guarded by another piece, since then that piece would be able to capture your king. What you're experiencing is called "scholar's mate", where the weak pawn on F2 (for white)/F7 (for black) is targeted- if they can get their queen onto that square, supported by another piece (usually a bishop), and you have no pieces other than the king defending that square, then it's checkmate. It's considered the weak pawn because at the beginning of any game, it is only defended by the king, whereas all your other pawns are defended by 2 pieces.

Don't worry about it, attacks aiming to checkmate you on this square (or at least win your rook in the corner) are one of the first things any beginner chess player needs to learn how to both do and defend against. Type scholar's mate into YouTube and you will find some really instructional videos which will tell you all about it. Learning to defend against it also teaches beginners a really important principle in chess, which is that you always need to look at where your opponent wants to go on their next move, before you prompt them to move away from where they are- or you might be prompting your own demise!

As you're a beginner, don't forget to watch a video on the special moves called castling and castling long, including the rules about when you are allowed Vs not allowed to do them

Please, do yourself a favor and stop playing bullet games!

You will not make any progress if you keep playing bullet. If you make a move after just a couple of seconds each time, you are just making random moves, shuffling pieces around.

An example from your game vs Meenaa89

Your opponent threatens checkmate, while simultaneously attacking your Knight. The checkmate is more important, you should make sure you see that. The attack on your Knight is also worth noticing. But you won't see either threat: You only spent 1.4 seconds on playing c6!

At your level, there is no way you can find the best move, or even a reasonable move, in less than 2 seconds. Even players with 6 times your rating, with hundreds of games more experience than you, can't pull that off. (unless they have seen the positions many times before).

You should play 10 minute games or preferably even longer.

You will not play good moves if you skip the part where you think about which move to play. For every move you play, you need to look at the board, see what your opponent might be trying to do, identify all moves you can play and then determine which one is best. I wrote a (very) simple guide. Finding the best move takes time.

Fwiw I do agree with duckfest. Rapid is for learning, blitz and bullet are for sharpening your response time.

Dont play bullet to start. You will never get better at chess playing bullet. 30 minute rapid only until you hit 1200 which once you reach 1200 you are no longer a beginner.

Now that you are playing 30 minute games you will have plenty of time to defend. They are most likely opening with queen wayward attack. That isn't good for white as its very easy to defend. You just have to slow down and think.

https://www.youtube.com/watch?v=2a3wDY69NJI

@gentle_please

read this... it's easy....it will answer your questions....

Bobby Fischer Teaches Chess...

https://www.chess.com/blog/RussBell/bobby-fischer-teaches-chessa-book-review

https://www.chess.com/blog/RussBell

From what I see, your blog post and Fischer's book are way, way, way too advanced for the OP. He needs some source to explain what "check" is, what "checkmate" is, and things like that a king can't move into check.

If you can give us some examples of the games where you got checkmated in that way, we are happy to give you a feedback. Concrete examples are the best.

Chess might not be the game for you. Unless you're completely new. In which case,

https://www.chess.com/lessons/winning-the-game/the-4-move-checkmate

There is also the queen's frenzy. Which youtuber Levy has made a guide on.

https://www.youtube.com/watch?v=cY9zitJFglc&t=1484s&pp=ygUYZ290aGFtIGNoZXNzIGVhcmx5IHF1ZWVu