So, you know how we have mountains in the United States? Well, there are some mountains that are really, really far away from other mountains. We call those "isolated major summits."
A "table" is kind of like a list, but it's set up in a special way. In this case, the table of the most isolated major summits is a list of mountains that are really far away from other mountains, and they're organized from the most isolated to the least isolated.
Basically, it's a way for people who study mountains to figure out which ones are the most special!