What does the prefix cis- indicate?
It basically means "functional groups are on the side side of the carbon chain of the molecule".
It's used in naming geometric isomers, which are pairs of molecules that have the same chemical formula but where functional groups can be rotated into different orientations when the molecule is considered in 3 dimensions.
"Cis" denotes functional groups on the same side of the carbon chain, whilst "trans" denotes functional groups on the opposite side.
You can see various examples here http://www.chemguide.co.uk/basicorg/isomerism/geometric.html.