You have to first understand that a structural formula can be given for propyl, not 5-propyl. The number 5 written before propyl that you may have seen in the chemical formula of several organic compounds indicates where the substituent propyl is attached to the base carbon chain.
Propyl is an alkyl with 3 carbon atoms. It is the substituent form of propane that is created when one hydrogen atom is missing from propane; this gives propyl a chemical formula of C3H7 instead of C3H8 which is the chemical formula of propane.
When the propyl substituent is linked to the base carbon chain with the carbon atom at the end it is called prop-1-yl and when it is linked with the carbon atom in the middle it is called prop-2-yl or methylethyl.