A barn was a farm building.
As a child, the First Doctor often went to sleep in a barn on Gallifrey. (TV: Listen) The Doctor later returned to it with the intention of activating the Moment (TV: The Day of the Doctor) and upon escaping his confession dial. (TV: Hell Bent)
Richard Mace took the Fifth Doctor, Adric, Nyssa and Tegan Jovanka to a barn after they were attacked by the villagers. (TV: The Visitation)
Liz Shaw was attacked by a Silurian in a barn. (TV: Doctor Who and the Silurians)
Sarah Jane Smith ran to a barn whilst being chased by Madra, who was impersonating Harry Sullivan. (TV: Terror of the Zygons)