The visual of the group is actually an official position in idol groups. The visual is pretty or handsome and they are basically expected to be a professional model to gain more fans for the group.
They are often positioned in the center or slightly off-center throughout most dance routines, and music show cameras tend to gravitate towards them. They are usually given more prominent positions in seating arrangements and standing placement during interviews.
They are sometimes cast in CFs or solo photo shoots for magazines.
When an idol introduces her/himself as the visual, they mean that they are a walking advertisement for their group.