The meaning of "plan B" is:
The definition of "plan B" is:
(noun) a plan in case something bad happens
A: I want to go to Harvard University to study medicine.
B: Do you have a plan B in case you don’t get in?
A: Yeah, I’m going to be a singer instead.
Father: What do you want to do when you grow up, son?
Son: I want to play guitar in a band and become famous.
Father: That’s very difficult. Do you have a plan B?
Son: Yeah, I want to marry a rich old woman!
The family made plans to go to Mexico, but had a plan B in case they had to cancel their vacation.