Keryo-Pente is just Pente except that in addition to being able to capture pairs of stones, you can also capture triplets of stones (in the same way, by placing a piece of yours on either side of the triplet). This blocks a common defense used in Pente for threatened pairs, which is to simply make triplets of them.
Because this makes it possible to capture stones fairly quickly, a capturing win (rather than a placement win) requires capturing 15 of your opponent's stones, rather than just 10 as in Pente.
Other than the above, Keryo-Pente is played exactly like Pente.
(This rules/instructions page covers Keryo Pente. Here at Pocket-Monkey, we also have straight Pente, Pro Pente, and Pro Keryo-Pente.)