The MCC Boarding Walker was a prototype boarding craft developed by the Republic during the Clone Wars.
When Anakin Skywalker used AT-TEs on asteroids against general Grevious and to board Cad Bane's frigate, Republic tacticians suggest that a boarding craft be developed. They finally were used by admiral Lobyc in the Battle of Firaca to do the same things Skywalker did.
MCC stands for Mobile Command Centre. This means that it can also be used on land for quick deployment. Its main cabin is outside to allow for troops to deploy faster. It has an energy bomb cannon that allows it to shoot explosive bolts of energy. It has mainly forward cannons to allow for a strong forward attack.