Exposure means the group sits face up in front of the rack, visible to the table. After an American pung, kong, or quint, that group is exposed.
An exposed group is no longer concealed. If the card wanted that group concealed, the line is broken.
Chinese tables call the same idea 明: a melded pung or kong is already shown.
American racks keep concealed tiles standing and exposed groups in front, so the table can see the difference at a glance.
After you call, expose it. Until it is shown, the group is not made.