Lindelöf's lemma
Lindelöf's lemma is a fancy math rule that says if you have a big collection of boxes (which we call sets), and each box is covering some part of a space (which we call a topological space), then you can choose just a few of those boxes to cover the whole space.
Imagine you have a big room with lots of toys scattered all around. You don't want to step on any toys, so you decide to put a box over each group of toys to keep them in one place. Now, you have lots of boxes covering different parts of the room. But, you don't want to trip over any boxes either. So, you take only a few of the boxes and use them to cover the whole room.
Similarly, with Lindelöf's lemma, you have a big space (like a room), and lots of sets (like boxes) covering different parts of the space. And the rule says you can choose just a few of those sets to cover the whole space, without leaving any uncovered parts. It's like you're making a puzzle out of sets, and they all fit together perfectly to cover the whole space.
Imagine you have a big room with lots of toys scattered all around. You don't want to step on any toys, so you decide to put a box over each group of toys to keep them in one place. Now, you have lots of boxes covering different parts of the room. But, you don't want to trip over any boxes either. So, you take only a few of the boxes and use them to cover the whole room.
Similarly, with Lindelöf's lemma, you have a big space (like a room), and lots of sets (like boxes) covering different parts of the space. And the rule says you can choose just a few of those sets to cover the whole space, without leaving any uncovered parts. It's like you're making a puzzle out of sets, and they all fit together perfectly to cover the whole space.