Question

What are cis and trans geometric isomers?

Answer 1

These isomers occur around double `C=C` bonds.

Explanation:

While everything around a single `C-C` bond is free to rotate, the double bond is not flexible. Now the attachments on the double-bonded `C`'s can be on the same side (cis-):

or on the opposite side of each other (trans-) :

Since there is no rotation possible around the double bond (in the middle) these two can not be converted into one another.

Extra
Instead of using the prefixes cis- and trans- the use of the prefixes Z- and E- are in common use.
The lower picture would be called trans-but-2-ene or E-but-2-ene
(and since the cis/trans can only happen if the double bond is at place 2, the 2 is often omitted, so trans-butene or E-butene)