Display dance generally refers to the sort of dancing that is watched by spectators. "Why has it been included in the 'Joining in' section?!" you may ask. Well, at risk of stating the obvious, it's because in order for there to be spectators, there need to be dancers to watch - and all the teams listed here are always keen for new members!
It won't usually matter if you've never done it before - teams usually practice weekly, and the more experienced dancers will be responsible for teaching the newer ones. You're usually only allowed to dance out once you're pretty good, so don't worry - you won't be forced to do it in public until you're ready to!
To find out more, contact the individual sides via their pages. There's a brief description of the various types of dance that can be accessed via the menu on the right, which should give you an idea of what you're looking at if you're not familiar with the terms.