"Prior to" means "before". But it sounds more formal. People use "prior to" in business situations, in a courtroom, and so on. There are some other differences between "prior to" and "before as well. You can say:
Before you go, can you take a quick look at this email I'm sending to Tracy?
But you can't replace "before" with "prior to" in that sentence. You have to follow "prior to" with a noun:
"Prior to departure" means "before you leave". The phrase "prior to ___" can also include a verb with the "-ing" ending:
Please call to confirm your flight details prior to departure.
Prior to meeting Vick, I had never even seen an opera, much less known an actual opera performer.