The airside is the part of an
airport where people go to board planes—in contrast to the
landside, where they may be doing other things. Once you show your
boarding pass to the
security personnel at
Checkpoint Charlie, you're in the airside.
The airside of an airport can follow many designs. Gatwick Airport and Miami International Airport have ridiculously long piers with moving walkways to help passengers get from one end to the other. At Dallas Fort Worth International Airport, the airside is on the outer edge of semicircular terminals, and thus only a few hundred feet away from the parking lot. Go to Pittsburgh International Airport, and the airside is in a separate building altogether.