The dancer of the group loves to dance. They spend lots of time in the practice room perfecting their choreography. They tend to move with a confident fluidity that brings a little something extra to idol stage performances.
The person designated group dancer is often in charge of monitoring the rest of the group during practice to make sure members are practicing the right moves while staying synchronized.
Many idol dancers also love to design choreography and are sometimes given a chance to choreograph for the group, which is always a source of pride for fans.
In interviews the dancer is most often called upon to demonstrate the point dance (포인트 안무) for the current song. They are often featured in dance breaks, which gives them a chance to show off their skills.
When an idol introduces her/himself as the dancer, they are telling you to keep an eye on them for the best dance moves in the group.