This depends on the time - mosaics were used in many different times in history.
For example, the Romans often portrayed their mythodology - check for example the attached roman mosaic from an Austrian museum - it portrayed the labyrinth and the story of Minotaur.
So a very common there was mythodologyOften, fruit or animals were portrayed. Sometimes - abstract symbols.
And yes, mosaic is still used today - I've seen it in an underground station in Vienna recently!