A square has no curves, only straight lines around its edges. Each of these surrounding straight lines is called a side. I know that if you are sliding your finger around each side at a time, it is tricky knowing when you have returned to the beginning, because they all look the same, don't they?
You could put a coloured sticker on one corner, start there, and you'd know when you'd got to the beginning. You will have counted four sides. Yes, that's right - 4. Every square must have 4 equal sides.