Explain why alkenes can have stereoisomers

"Stereo-" refers to 3D space, like with stereo sound involving sound from all directions.  The C=C double bond has no free rotation, as this would require breaking the pi bond, which is energetically unfavourable. This leads to the occurencce of stereoisomers whereby the constituents attached to the C=C can be in a different orientation in space, while maintaining the molecular formula. E (entgegen = german for opposite) and Z (zusammen = german for together) are the two isomers possible around a C=C. E is when the two largest constituents are across from each other ie bottom right and top left, and Z is when they are together ie bottom right and bottom left.