The play's title comes from a biblical passage that resonates throughout the play:
Judge not, that ye be not judged. For with that judgment ye judge, ye shall be judged: and with what measure you meet, it shall be measured unto you again. (Matthew 7:1-2)
In other words, don't be so quick to judge other people because nobody is perfect and everyone is subject to God's judgment.
We can see how this applies to Angelo, who condemns a man to death for "fornicating" and then, like a hypocrite, turns around and propositions Isabella.
The term "measure for measure" also refers to a legal concept that is central to the play. That is, when a person commits a crime (or sins), he or she should be made to pay – either by making some sort of restitution or by suffering an amount that's equal to the suffering he or she has caused. (This is similar to "eye for an eye" justice.)
You want an example? Of course you do. When Angelo sentences Claudio to death for having sex outside of marriage, the penalty definitely doesn't measure up to the crime.
Later, when the Duke sentences Angelo to death for what he's done to Claudio and Isabella, he declares "An Angelo for a Claudio, death for death [...] Measure still for Measure" (5.1.49). Even though he pardons Angelo in the end, the Duke's point is pretty clear – Angelo's punishment should be equal to the suffering he's caused.