To explain a racemic mixture it is firstly necessary to explain what an enantiomer is.
Enantiomers are stereoisomers that are non-superimposable mirror images of each other. An enantiomer has no plane of symmetry and contains a chiral centre.
A racemic mixture is simply a mixture containing an equal amount of each enantiomer.
Unfortunately it is one of those names that does not directly relate to what it is. The name racemic comes from racemic acid which was the first racemic mixture discovered.