They were called Scanbands, the implication was that you would scan them to access attractions. While they were told they were VIP's everyone else in the park had one as well so I would assume the Scanbands the main characters had were set up to give them better access or perhaps everyone was considered a VIP.
Anyway obviously the comedy in this movie came from making fun of many of the negatives of the theme park industry, long lines, overpriced sodas, cheap merchandise, inexperienced ride operators and of course they chose to include Magic Bands in with that. It was a good choice in my opinion!