If your last seed lands in an empty pit on your side, you take it and every seed in the pit opposite into your store.
A capture happens when the last seed of your sowing drops into one of your own pits that was empty. If the pit directly opposite holds seeds, you move those seeds and the one you just dropped into your store. If the opposite pit is empty, nothing is captured and the seed stays where it landed.
Both directions
Captures only work on your own side, but your opponent has the same threat. An empty pit of theirs facing one of your full pits is a danger, so look at it before you move. A capture ends your turn unless the sowing also finished in your store (another turn in Mancala).
Setting up a capture while guarding against one is most of the middle game. See the strategy guide and the basic rules.