Altium Schematics: what does Locked function mean?
In short

Parts inside component

A Component may have more than one parts, for ex: LM324 has 4 op-amps inside. With the part option, you select which one to use. Altium will create netlist for the pins with the part you have selected. 

Part locked

Now, when you annotate the schematic, Altium not only changes the component names with ? (for ex U?), but also the parts. This may cause trouble in PCB because it effects witch pins of the part is connected to the rest of the circuit. If you select "part locked", Altium will keep all locked parts with your selections. For ex: A component with locked enable, lets say U?/part2 will not change to U1/part1 when you annotate schematics. It will be something like Uxx/part2 always.