What does it mean when a triathlon has a team option?

Answer 1

Means that either you can enter as an individual, competing only by yourself. When you cross the finish line, you're done.

Or you can enter as a team, in which case you have some other racers with you. In that case the clock won't stop with the first person of the team across the line, but at a later stage. Some races are timed against the slowest member of the team, some after three out of four members have passed the line, and there might be other options used as well.