What is a censure?
The short answer
A censure is a Church penalty meant to bring about a change of behavior and reintegrate the offender into the Church. That is why it is called a medicinal penalty, as opposed to an expiatory one, which is compensatory.
A penalty with a purpose
A censure is a type of penalty imposed by the Church to motivate a change in behavior and bring the offender back into the Church.
Medicinal, not expiatory
For that reason it is called a medicinal penalty (CIC 1312), as distinct from an expiatory penalty, which is compensatory in nature.