Actus Reus is the guilty act part of an offence - the external element, for example kicking someone or taking something. Basically it's the thing you physically do when a crime is committed.

Mens Rea is the guilty mind part of an offence. This is the internal element and it involves what the defendant (the person who commited the offence) is thinking at the time. An example is whether the defendant intended to commit the offence or what they believed the circumstances were.