Wiktionary
second unit
n. (context cinematography English) A separate team that shoots footage that is of secondary importance for the final cut of the motion picture, as opposed to first unit.
Wikipedia
Second unit
In film, the second unit is a discrete team tasked with filming shots or sequences separate from the main, or "first", unit. The second unit will often shoot simultaneously with the first unit, allowing the filming stage of production to be completed faster.