Capitol Hemp sells hemp-related products. Hemp is a variant of the cannabis plant that is grown not for ingesting as a drug, but for mostly industrial purposes; making ropes, fabric, soaps, oils, and a bunch of other stuff. The store also carries water pipes, vaporizers, rolling papers, and other retail products related to smoking, but no you can't smoke there, nor purchase weed for smoking there.
Marijuana is partially decriminalized in DC, but you can't buy it at Capitol Hemp or any regular retail store. If you want to legally exchange money for marijuana in the District, you need a physician to write a prescription and a licensed dispensary to make the actual transaction.
Capitol Hemp is located at 1770 Columbia Rd NW and is open daily.