(m) means that a noun is masculine. Spanish nouns have a gender, which is either feminine (like la mujer or la luna) or masculine (like el hombre or el sol).
The fabric along the selvedge won't fray if you pull on it, but the perpendicular edge will.La tela que está a lo largo del orillo no se va a deshilacharse si jalas de ella, pero con el borde perpendicular, sí.