The TransmogriShield allows you to use classic Arduino shields on the Arduino Leonardo. Because of the new ATmega32U4 microcontroller used on the Leonardo, some serial pin assignments (i.e. SPI and I2C) were changed, which meant the Leonardo could not use many shields designed for Arduino Uno or others that used the ATmega328 microcontroller.
By using an arrangement of surface-mount headers to "intercept" the incoming pins and shift them around, the TransmogriShield takes the SPI and I2C lines from the Leonardo's positions (at the ICSP header and D2/D3 pins) and moves them back into classic Arduino positions: SPI on D11-13 and I2C on A4/A5. Any shield placed above the TransmogriShield in the stack will see those lines in their old positions.
As this shield was designed with legacy shields in mind, the female sockets have fewer pins than the R3-compatible headers on the Leonardo. Instead of one 6-pin, two 8-pin and one 10-pin connector (which the male headers have for connection to the Leonardo), the female sockets have the legacy pair of 6-pin and pair of 8-pin connectors.
Along with allowing you to use classic SPI/I2C-based shields, the TransmogriShield provides plenty of prototyping space, so it also makes a pretty good protoshield for the Leonardo.
Arduino Leonardo Shield Adapter Resources