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Perimeter is the distance around a shape. To find the perimeter of a rectangle, add all four side lengths. A rectangle with sides of 3 and 2 units has a perimeter of 3+2+3+2 = 10 units.
Perimeter is simply the total distance you would walk if you traveled all the way around the outside of a shape without lifting your feet. Imagine walking around a soccer field: you'd walk one long side, one short side, the other long side, and the other short side. Add those four distances together, and you have the perimeter of the field.
A rectangle has a special property that makes perimeter easy to work with: opposite sides are equal. The two long sides are the same length, and the two short sides are the same length. So if you know the length and the width, you know all four sides. A rectangle 3 units long and 2 units wide has sides of 3, 2, 3, 2 — and the perimeter is 3 + 2 + 3 + 2 = 10 units.
You can also think about this using what you know about addition. Since the two lengths are the same and the two widths are the same, you're really adding pairs: 3 + 3 = 6 for the two long sides, and 2 + 2 = 4 for the two short sides, then 6 + 4 = 10. Either way of adding gives the same answer.
One thing to pay close attention to is the units. If the sides are measured in centimeters, the perimeter is in centimeters. If the sides are measured in feet, the perimeter is in feet. Perimeter is a length — a distance — so it always needs a unit. Writing "10" without "units" or "cm" or "feet" is an incomplete answer.