Barbwire, or barbed wire, was strong wire with sharp metal spikes running along it, and used in fencing.
In 1885, barbwire was fairly new and wouldn't become a common sight for some time.
A salesman selling this then new-fangled product had been traveling all over the country by the time he reached Hill Valley. He carried a sample of barbwire with him to show potential buyers.
Whether the salesman managed to sell any barbwire in Hill Valley before he departed on the train for San Francisco with a friend — a trip that was cut short twice, first due to Clara Clayton pulling the emergency cord, and then Locomotive 131 being hijacked by “Clint Eastwood” and Emmett Brown — remains unrecorded.
Barbwire was also used atop the chain-link fence surrounding the burnt-out remains of Hill Valley High School in 1985A, which also bore a NO TRESPASSING sign from the Hill Valley Police Department.