The way I've seen it explained in popsci books is like a long fishing line stretched between two points. When you (a large human) look at it you see it as 1 dim

The way I've seen it explained in popsci books is like a long fishing line stretched between two points. When you (a large human) look at it you see it as 1 dimensional, you can go either forward or backward. But now imagine you are a tiny bacteria moving along it. You see it as a 2D surface, you can go forward or backward along its length as initially described, but you can also make a 90 degree turn and move around the circumference of the fishing line. The 2nd dimension is there but it's "tiny and curled up". Not sure how well that analogy tracks the extra tiny dimensions of string theory, but it has stuck with me.