When you are running a simulation using the Analysis - Simulate command, the Sonnet Cadence Virtuoso Interface is locked until the analysis is complete. You may not make any changes or execute any commands in the interface until it is unlocked when Sonnet is closed.
The Analysis - Unlock Project command unlocks a Sonnet Project if, for some reason, it did not complete simulation and failed. The interface is locked up, and the user will have to Abort Simulation from the menu (Analysis Abort Simulation) or by clicking on the Abort Simulation button in the main interface window, then unlock the project.