MAD (a.k.a Mid-Acts Dispensationalism) is the recognition that in the Bible God works with people in different ways. Some people like to refer to this as God "changing the rules".
For example, at one time God had mankind eat only fruits and vegetables. Later, after the Flood, mankind could eat meat.
At one time, to be saved, certain people had to build a boat (Noah and his family). However, now we don't have to build boats to be saved. One could say that it is Biblical to build a boat to be saved...but not for the current dispensation.
So throughout the Bible God works with people in different ways.
Abraham had to circumcise his descendants and that circumcision became part of the Law. The Hebrews had to obey the law.
When Jesus came He came under the dispensation of the Law. He taught the Law, lived by the Law, and taught the 12 to teach the Law. Jesus taught the Gospel of the Kingdom (which was prophesied), which is different than the Gospel Paul preached.
When Paul was saved (About MID way through the book of ACTS...Hence Mid Acts dispensationalism) a new dispensation was "created". Paul taught the revelation of the Mystery (which was not prophesied). Part of this mystery was that people are no longer under the law, no longer had to be circumcised, no longer had to be baptized, etc... Salvation comes by
believing.
That it is in a nutshell (as far as I understand it).
You might also say it is the difference between Prophecy and Mystery.
