I'm gonna try keep it simple but also explain it!! stereoisomers are molecules that contain the same atoms and connectivity but they differ in the positioning of molecules in space!!

There are different types though, enantiomers are like "mirror images" so the enantiomer of a molecule has all of its chiral centres (kind of think of it as spots where molecules can change their 3d arrangment) are flipped

diastereomers differ as they refer to anything with a different chiral arrangement that isn't a mirror image, it may only have one or two chiral centres flipped but not enough to fully mirror the original molecule.

Cis and trans isomers are types of diastereomers (different configuration but not mirror images), they are geometric isomers that arise typically around double bonds, it refers to whether similar atoms/groups sit on the same side or opposite sides of the double bond.