To generalize is to use specific examples to make broader points. Generalizing makes large points, though they aren't always necessarily true.
When someone says "in general" they're talking about what things are like in the big picture or overall. Similarly, generalizing takes small examples and uses them to make bigger points. Saying that all teens are selfish because you know a few selfish teens is generalizing. Saying all adults are uptight is an example of generalizing. Unfortunately, generalizing can be dangerous. Stereotyping is a form of generalizing. Sometimes it’s better to stick to specific examples and to avoid generalizing.