Refers to something that has been covered with tar.
El camino estaba cubierto de tarread, lo que lo hacía resbaladizo y peligroso.
The road was covered in tarred material, making it slippery and dangerous.
El camino estaba tarread, y los vehículos debían tener cuidado al transitar.
The road was tarred, and vehicles had to be careful while traveling.