The black queen has just landed on g2, right next to White’s king. She is checking him, she is defended by nothing, and she has one move left in her life.
g There is a nice consequence hiding in that position. A queen standing next to a king always attacks it, because the king is one square away in some direction and she covers every one of those. So any time your king can capture a queen, she was giving check, and the capture is not a bonus. It is your way out of the check.
The one thing that stops him
She has to be undefended. If a bishop, a knight, a rook, a pawn or her own king covers g2, then Kxg2 would put the white king on a square the defender attacks, and a king may not move into check. The move is simply not available.
Watch out for defenders you cannot see in one glance. A black rook on g8 with the g-file clear defends g2 from six squares away. A bishop on b7 defends it from the far corner. That is the whole trick behind the shortest mate in the book.
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a g That picture has a name, the kiss of death, and it is worth burning into memory from both sides of the board. Queen next to king, queen protected, king boxed in by its own pawns. Mate.
What this costs beginners
Roughly one queen a week, in my experience with juniors. The pattern is always the same: they find a check, they play it because it is a check, and the queen is sitting next to a king with nothing behind her. The opponent takes and the game is over.
So before you put your queen on a square touching the enemy king, ask one question: what defends her there? If the answer is nothing, you are not giving check, you are giving away nine points. Queens dropped this way are just hanging pieces with a crown on. And if the king is the one doing the taking, remember he can eat anything undefended nearby, except the other king.