I understand most of it, but what's still bugging me is the wording for the last part here.

For example, all characters in the Legion of Super-Heroes Starter Game have the “Legion” keyword. In a game with a 300-point build total, you could pick any of the characters in the set (they all have the same “Legion” keyword among other, different keywords), but you would need at least three characters (to meet both the three-character minimum and the one per 100 points’ requirement) to play a “Legion”-themed team.

What does that mean?

Say I have Jean Grey 31, Psylocke 45, and Havock 44. And Wiccan and Iron Widow to fill out the rest. Would they count as a X-Men theme since I have 3 figs?

To answer your last example, no (even though we don't know the said X-Men team members' keywords yet). All team members must have the same key word. I skipped that qualification too when I first read it, but it is in the Theme Team description. If only Jean, Psylocke and Havok were to have the X-Men keyword (let's say they were to only have the X-Men keyword, for this example). Wiccan and Iron Lad don't have the "X-Men" keyword. So, they would not be a theme team.

As for "what does this mean" part. It means this: if you played a 300 point game, the number of figures you need is 3. If you played a 400 point game, the number of figures you need is 4, and so on up. Now, if you played a 200 point game, you will still need 3 figures to satisfy the minimum 3 figure team; and 3 for a 100 point game.

All characters have to have the same keyword and you need 1 character for every 100pts of team build. For example, a 300pt game would require 3 members to be awarded the theme team bonus...all with the same keyword.