Brown aprons are used to enter the Crafting Guild. You can buy them from Thessalia's Fine Clothes in Varrock for 2 coins.
It can also be received from Malignius Mortifer, who is located north-east of Rimmington after the Swan Song quest.
If you have a level of 35 in construction, you can obtain a brown apron if you build a Tool Store 3 in your home.
It is also used in a medium level clue.