The parent chain is the chain containing large amount of carbon atom.
Thus,
CH3-CH-CH3 is the parent chain.
Thus, the number of carbon atoms in this chain is three.
The substituent group is the another group of chain attached to the parent chain containing less carbon atoms. The substitutent group is attached to the second carbon of the parent chain.
Thus, the substitutent group is 2-ethane (CH2-CH3).