Perimeter answers one plain question: how far is it all the way around the outside of a shape? Imagine an ant walking the whole edge and back to where it started — the distance it covers is the perimeter.
Adding, One Side at a Time
A quadrilateral has four sides, so the ant walks four straight bits. To find how far that is, you just add the four side lengths together. Hit "walk around the edge" in the toy and you'll see exactly that — each side lights up as the ant crosses it, and its length gets tacked onto a running total. By the time you're back at the start, you've added all four, and that total is the perimeter.
So There's No Real "formula"
People sometimes write rules for special shapes (you'll meet the rectangle shortcut next rung), but underneath, every one of them is the same move: go round the outline and total the edges. That's why perimeter never trips you up as long as you account for every side — which becomes the whole game once shapes get bendy.
Why It's Measured in Plain Units
You're measuring a distance — a length you'd walk or a fence you'd buy — so the answer is in plain units like centimetres or metres. There's no little ² here, because you aren't covering a flat space (that's area, a different idea). Perimeter is the trip; area is the floor.