The number indicates how many people are at the table. A "4 top" is 4 people at a table, and pretty standard. A "10 top" is not only 2.5x as many people as normal, it's also chaotic to seat and serve normally. You will also generally have to split out bills, some times in a complex way. Worst of all, the size of the table diffuses responsibility for tipping, generally leading to paltry or no tips. This will occasionally go the other way, with someone being generous as it's usually an event, but the "5 minutes before close" crowd doesn't tend to be generous.

So it's a hard table, that won't pay well, right before you were about to go home for the night.