Backup
Reserve, Spare, Copy, Stub
noun
Something that can abstain until it's needed makes for good backup. Think about it — you don't want backup rushing in too early, right? It should abstain until the backup is actually needed.
Note that 控え isn't used for spares and extras. It's more for necessary backups, like a backup player in sports, a document copy for record keeping, or a receipt stub.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You learned this reading already! Good for you!
控えの選手には誰がいたっけ？
Who’s the reserve player?