PDEs and Characteristics

[Nebula]

Hello.
I just wanted to know the exact definition of a characteristic. I know that they are lines on which the solution is constant for a homogeneous solution and behaves in a certain way for an in homogenious equation.
I have a question in an assignment wanting me to prove that the solution is constant along a characteristic. This suggests to me that my understanding of characteristics is not quite right as proof by definition seems unsatisfying.

Uhm this is an applied math course so I do not need something rigorous.

Thank you for you time.