Because the new employee probably still doesn't know right from left (doesn't know anything), senior employees need to tighten the reins and guide him properly.
❓ 確認クイズ問題
🎉 クイズ解説
English Explanation
"右も左もわからない" means knowing nothing about the surroundings. "手綱を締める" means to tighten the reins / control strictly.