1.5 oz Fiberglass Mat - 38" x 20 yd

Chopped strand mat consists of randomly oriented, chopped glass fibers, bonded together using a polyester binder.
When saturated with resin, the binder dissolves and the fibers have the ability to be moved around. This makes it easier to conform mat to tight curves and corners than woven fabrics.
Matting is compatible with unsaturated ortho-polyester, iso-polyester, vinyl ester and a variety of other resins.
The polyester binder makes this fabric incompatible with epoxy resins.

Being relatively inexpensive, mat is commonly used for building thickness in diverse fiberglass structures, such as mold construction, panels, plywood decking, fiberglass furniture, and between layers of fabric when molding storage tanks and boats.
Chopped strand mat is also used as a backing for gelcoat, as it has less pattern transfer than most woven fabrics — so is less likely to show through the gelcoat.

Having a high resin to glass ratio in finished products, chopped strand mat does not have as much strength as woven fabrics. (It's the glass that makes fiberglass strong.)
If you need strength you should choose a woven cloth, or you could mix the two.
Mat can be used between layers of woven fabric to help build thickness quickly and aid in the bonding (with that high resin content) of woven layers.