From YPPedia
Note: The chain is a similar concept, often confused with combo.

A combo, short for combination, is the act of getting two or more clears simultaneously. They are very important in bilging, but less important in swordfighting, navigation, and sailing. It is also quite important in alchemistry. Notices for more valuable combos are larger than those for less valuable combos.

It is important to note that the word 'combo' is almost always used to describe a type of swordfighting attack despite the correct term being a 'chain'. As stated above, the only puzzle for which combos are vastly superior to and more desirable than chains is bilging.