The two structures shown (left and right) are stereoisomers of 4-bromopent-2-ene, $\mathrm{CH}{}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CHBr}-\mathrm{CH}{}_3$. They are related as

The compound has two stereogenic units: the C2=C3 double bond ($E$ or $Z$) and the stereocentre C4 ($R$ or $S$). The left structure is ($E$,4S) and the right one is ($Z$,4R). The mirror image of ($E$,4S) is ($E$,4R), because reflection inverts stereocentres but does not change double-bond geometry. The right structure has the $Z$ double bond, so it is neither identical to the left one nor its mirror image: the two are diastereomers.