A blanket statement is a sentence that states something about everyone.
A blanket statement can be seen as mean spirited and not as well thought out, which means making blanket statements takes away the uniqueness of every other person. This is why it’s important to keep one’s opinions in line with what they’re discussing, rather than generalizing everything to make an example fit. If someone were claiming that people should have more knowledge on certain topics, or should have more responsibility in their life decisions, then using a blanket statement like “everyone” could make their opinion seem invalid because they imply there are no exceptions–a lot of people feel that way too but don’t need or want to be part of the discussion because it doesn’t affect them.