A capture jumps over an adjacent opponent onto the empty square immediately beyond it. In English/American checkers a man captures forward. If any of your pieces can jump, you must choose a capture instead of an ordinary step.
Worked example
Imagine red on 10, white on 15 and an empty 19. Red can jump from 10 over 15 to 19. If 19 were occupied, that jump would not be available: an opponent to jump over is not enough without an empty landing square.
One Piece Can Jump
Red men are on 9 and 12. White men are on 14 and 28. Every other square is empty. Red moves toward the top and it is red's turn. The man on 12 has an empty diagonal neighbor, but first check whether the man on 9 can capture.