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Perimeter is the total distance around a shape. You find it by adding all the side lengths. Perimeter is different from area; a shape can have the same area as another but a different perimeter.
Trace the outline of shapes with your finger while counting. Place a string around a shape and then measure the string's length. Compare areas and perimeters of different rectangles.
You already know how to measure the length of a single straight line using a ruler. Perimeter simply applies that skill to the boundary of a shape — instead of measuring one line, you measure every edge and add them all up.
The most helpful way to understand perimeter is as a walk. Imagine you're an ant starting at one corner of a rectangle. You walk along every edge until you return to where you started. The total distance you walked is the perimeter. The shape doesn't matter — triangle, square, hexagon, or any polygon — the perimeter is always the sum of all the side lengths.
For a rectangle with sides of 5 cm and 3 cm, the ant walks: 5 + 3 + 5 + 3 = 16 cm. Rectangles have two pairs of equal sides, which is why you add each length twice. But for irregular shapes, just add every side individually — there's no shortcut required, just careful counting and addition.
Here's the concept that surprises many students: perimeter and area are completely different measurements of the same shape. Area measures how much surface is inside; perimeter measures the distance around the outside edge. A long, skinny rectangle (like 1 cm × 10 cm) has an area of 10 square centimeters — but so does a square that is about 3.16 cm on each side. Yet their perimeters are very different: 22 cm versus about 12.6 cm. This means you cannot determine one from the other without more information. Keeping the two ideas separate in your mind — inside vs. boundary — will serve you throughout geometry.