To "assume" that something is true means that you think it's true, without having any real proof or evidence.

"Generally" means "mostly" or "usually".

The phrase "people generally assume that ___" describes something that most people think is true, but may or may not actually be true. For example:

People generally assume that I'm in my thirties when they meet me, but I'm 48.

People generally assume that kids who grow up rich are going to turn out to be spoiled brats. That's not always true.