All cats are tabby cats. All tabby cats have fur. Therefore, all cats have fur. If the premises are true, the conclusion absolutely must be true.
All cats are animals.
All lions are animals. Therefore, all lions are cats. For an argument to be valid, it must be the case that IF the premises are true, the conclusion must also be true. But just because all cats are animals and all lions are animals doesn't mean that it's got to be true that all lions are cats. In some other possible world, lions could actually be members of the dog family.