moc is a command line tool included with Qt. Typically, the programmer does not interact directly with moc. Instead, the project Makefile generated by qmake will specify the needed moc actions for a given project.
moc stands for meta-object compiler and must be run on all classes in a project that descend from QObject. The result of running moc on a class is a new file with prefix moc_ that is pure C++.
moc is responsible for generating the code for implementing signal and slots, creating the Qt property system, implementing the Qt introspection mechanisms, and/or anything else that's suggested by a class' descending from QObject and using the Q_OBJECT macro.