What does P(A U B) mean?

U means    ' Union'

the upside down U  means 'Intersection'

Also...

 

For these,

 

You can replace  U  with  "or"

And replace  ∩  with  "and"

 

For example...

 

P(owns a PS4  U  does not own a cellphone) is the probability that a randomly selected person in the group owns a PS4 or does not own a cellphone.