In fact, any time you see a component symbol drawn with a diagonal arrow through it,
that component has a variable rather than a fixed value. This symbol ”modifier” (the diagonal
arrow) is standard electronic symbol convention.
Variable resistors must have some physical means of adjustment, either a rotating shaft
or lever that can be moved to vary the amount of electrical resistance. Here is a photograph
showing some devices called potentiometers, which can be used as variable resistors: