reserves; reserve army
Primarily military: troops held back for later use. Also used in organizational contexts for backup personnel.
The reserves were sent to the front line.
potential members; people at risk; wannabes
Figurative extension: people who may join a group, develop a condition, or aspire to something. Often used in social or medical contexts.
He is standing by as a reserve.
The elderly are said to be at risk of heatstroke.
They are potential future executive candidates.
industrial reserve army; reserve army of labor · the unemployed; jobless masses
Compound of 予備 (reserve, spare) and 軍 (army, troops). The figurative sense extends the military meaning to any group of potential members.