There can be a lot of meanings for isomers. In this case, we are showing the structural isomers of C₇H₁₆. Based on the chemical formula, CₓH₂ₓ₊₂ it is an alkane. They only differ in the positions of methane branches in the parent carbon chain. Basing on the attached picture, the parent carbon chain is pentane for both isomers. But the methyl branches are on the 2nd & 4th, and 2nd & 3rd carbon for 2,4 - dimethylpentane and 2,3 - dimethylpentane, respectively.
