K is proper cone. K is pointed?

Question

In Boyd-Vandenberghe Convex Optimization and had a question about the proper cone. The following is the property of a proper cone:
• K is convex.
• K is closed.
• K is solid, which means it has a nonempty interior.
• K is pointed, which means that it contains no line (or equivalently, x ∈ K, − x ∈ K =⇒ x = 0).
A proper cone K is pointed (which means that it contains no line). Can somebody please elaborate on what "it contains no-line" mean? P.S. I am new here