Snapback is an urban slang term for an adjustable flat brim baseball cap, which are popular in the hip-hop community. They are called a "snapback" because the back of the hat can be adjusted with different settings of the snaps.
Some lyrical examples of snapbacks:
" by Hollywood Undead ("Cruise through your valley and I'm snatchin' your snapback. Huffin' on some paint and I think that I'm half black.")
"Snapbacks & Tattoos
" by Driicky Graham (Snapbacks and tattoos. Nice whips, fly chicks. All that cause cash rules.")
" by Kid Cudi ("Cleveland City snakeskin snapback. Ni--as know that uniform, keep the brim low.")
" by Clean Bandit ("GRRRL PRTY is the label on my snapback. Doin' worldwide shows in a black hat.")