Mens rea is a Latin term that means, "guilty mind."
In the context of the law, mens rea refers to the burden of the prosecution to prove that the defendant not only committed the crime, but also that the person had the intention to commit the crime, and knew exactly what they were doing.
Mens rea is only required to be proven for certain convictions.
There are some circumstances under which a person may accidentally commit a crime without intending to. For example, a person may walk out of a bank with an envelope filled with thousands of dollars in cash that isn't theirs. With mens rea, or a guilty mind, this could constitute grand larceny. However, without mens rea, it could simply be an accident, and the person could be completely innocent. It is for such reasons that mens rea is required in a number of different conviction types.