reserves; reserve army
Primarily military: troops held back for later use. Also used in organizational contexts for backup personnel.
予備軍が前線に投入された。
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.
高齢者は熱中症の予備軍と言われる。
The elderly are said to be at risk of heatstroke.
彼は予備軍として待機している。
He is standing by as a reserve.
彼らは将来の幹部候補の予備軍だ。
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.