Okay kiddo, imagine you have a toy box with all your favorite toys inside. Now, if you take out all the toys, what do you see? You see the empty toy box.
Similarly, when we talk about a complete space in math, it means that there are no "holes" or "missing pieces" in that space. Just like the toy box is complete when it's all filled up with toys, a complete space in math is filled up with all the points that should be there.
Now, you might be wondering, "But how do we know what points should be in the space?" Well, mathematicians use some clever tricks and rules to figure that out. By making sure that every sequence of points in the space has a limit point (a point that the sequence gets closer and closer to), we can be sure that the space is complete.
So, to sum it up, a complete space in math is just like a full toy box with no missing toys. We can tell it's complete by making sure all the sequences in that space have a limit point. Does that make sense, kiddo?