In this module, you can study the meaning of "perimeter" and how to find the perimeter of a square, rectangle or triangle.
The video above is also shown under the "Area of squares, rectangles and triangles" page. It contains a 12-minute long explanation from Khan academy covering the basics of area and perimeter. This is by far the best video I could find using the metric system, as it does a good job of explaining the meanings of both area and perimeter and the differences between the two. However, Khan does make two mistakes in the video. See if you can spot them! (If not, read the description of this video on the "Area of squares, rectangles and triangles" page.)
The perimeter of a rectangle is NOT the length times the width. (That's the area, not the perimeter.)
You do not find perimeter by counting squares around the outside of the shape.
Study and practice
Use these search terms to find additional resources:
how to measure perimeter
perimeter of square
perimeter of rectangle
perimeter of triangle