It means "too" (too much mostly). But I recommend using the synonym - "слишком". Or "чрезмерно" at least.
EXAMPLE: "Штаны были либо слишком велики, либо чересчур малы". (The pants were either more than necessary, or less than necessary).
If someone told you "Это уж чересчур!", it seems that he doesn't like what you told him or did.