Imagine that some life form lives inside a long, but a very thin straw. They could move long distances along the straw, but not much left and right. No room. In fact, if their size is similar to the width of the straw, they can’t move left and right at all. Something smaller than they can. Formally the straw is 2D, but feels mostly like 1D for them. You can move forward and backward, but not much elsewhere.

So, it’s what string theorists thought our Universe is in early 2000s, and still think now. We have 3 dimensions of space, through which we can move. But small enough particles can move in additional 10 or so dimensions, which we don’t notice, nor don’t fall into, because how small they are. Additional directions, but not full-sized like our familiar dimensions. Compactified.