Piping is a narrow ribbon with a cord through it on one side. This side is the part that is visible when your project is finished. 
You use the piping as a decorative touch between your seams. It is nice to use one of the colours, but you can also use a contrasting colour. 
A nice detail with your clothes, bags or cushions for example!

Stitching your piping in three steps. 
First, stitch the piping to the right side of one of the fabrics.
Then pin the other fabric on top (right sides together).
Then turn the whole thing over and stitch right in the stitching from step 1 or, better yet, a little to the left of it.