The MV-Transformer is used to turn High Voltage (HV) EU current into Medium Voltage (MV) and back. This is useful for powering advanced machines and for medium-range energy transport. In order to power tier 1 machines like the Macerator, you will still need a LV-Transformer.
Normally, the MV-Transformer takes an input up to 512 EU/t on the face with three dots, and outputs 128 EU/t on any face with one dot. When the transformer is receiving a redstone signal, this is inverted: The three-dot face will output 512 EU/t, and any of the one-dot faces will take an input up to 128 EU/t.