what are cis-trans isomers ?

These are geometrical isomers for which molecular and structural formula are same but the spatial arrangements are different.
If the same groups are present about a center(which is the cause for this different arrangement) then the molecule is cis and if they are different about that center, it is called trans.You should go through any textbook fr details and if you would have any problem in understanding, you can post your specific doubts on the forum.