Side
noun
One of the kanji meanings is side, and as a standalone word this also means side, as in the sides of a shape.
へん
This has the same reading as the kanji you learned, meaning you know the reading!
この正方形の辺の長さは、それぞれ四センチです。
The length of each side of this square is four centimeters.
この三角形の辺の長さをモノサシではかってみてください。
Please measure the lengths of the sides of this triangle with a ruler.
これは二等辺三角形です。
This is an isosceles triangle.