中 is a suffix which means "in the middle of ~ing". The phrase ~中 functions as a noun.
Action nouns (e.g., りょこう[travel], 運転[drive]) are often used before 中.
今、父はりょこう中です。
My father is on a trip now.
使用中のコンピュータにさわらないでください。
Please don't touch a computer that is in use.
運転中におさけを飲んではいけません。
Don't drink alcohol while driving.