When the king is under attack, it is called check. If the king cannot escape the attack in any way, it is checkmate and the game ends: the side that mates wins.
Look at the three pawns in front of the black king. They are there to protect the king, but they also block its escape squares.
If the white rook goes to the eighth rank, the black king can't escape, nothing can block, and the rook can't be captured. Try it: mate in one.
Queen and king together are very strong. The white king on b6 controls a7 and b7. Bring the queen to the right square and mate.
Now rook and king. The white king on c7 controls b7 and b8. Find the move that mates with the rook.
When looking for mate, ask three questions: Is the king attacked? Does it have a square to escape to? Can the attacker be captured or blocked? If all three answers are no, it's mate!