Phat pants or phatties are usually made of denim

Denim

Denim is a rugged cotton twill textile, in which the weft passes under two or more warp threads. This produces the familiar diagonal ribbing identifiable on the reverse of the fabric, which distinguishes denim from cotton duck. Denim has been in American usage since the late 18th century...

 but can be made of any material, and are fitted at the waist, but get wider down the legs all the way to the ground where they enclose the feet due to their width. Phat pants are usually worn by ravers and can be used as a visual identifier. Ravers will often customise their phat pants by covering them with reflective materials of their own design.