To provide a lining or another layer on s.t.; to double (s.t. usually of the same kind so as to make it thicker).
The -an imp. focuses on the layer that you are adding s.t. to. The pa--un imp. focuses on the layer that is added to s.t. The pag--un imp. focuses on all the layers or strands that are being connected together.
Take care in making the bottom layer of your mat.