Imagine you and your friends build a castle out of blocks. You can move the blocks around to make different shapes, but no matter what you do, the castle always stays in one piece - it's connected.
Well, mathematicians like to play with blocks too, but instead of blocks they use something called "schemes" which are like shapes made out of points and lines. Grothendieck's connectedness theorem tells us that, if we have a scheme that is "nice enough" (meaning it follows some rules), then it will always be connected too!
Think of it like this: if your castle is made out of enough blocks and you follow the rules of building (like not leaving any holes), it will always stay whole. In a similar way, if a scheme is made up of enough points and lines and follows certain rules (like being "reduced" which means it doesn't have any unnecessary points), it will always be connected.
Grothendieck's connectedness theorem helps mathematicians study and understand these schemes, and see how they fit together like puzzle pieces to make bigger mathematical ideas.