The Force column shows whether the signal is currently forced, and to what value. If this column is empty for a signal, then the signal is not being forced, otherwise, it is forced to the shown value.
Forcing means that you override the simulator's computation of a signal and forces it to a specific value. All signals that depend on the forced signal are affected accordingly. It is typically used on input signals in order to see how the model reacts to specific input values, but it is also possible to force local and output signals. Forcing applies immediately to the last cycle of the simulation data and to all cycles being simulated while it is enabled. However, it is not possible to go back and force signals in cycles before the last cycle.
As an example, let's say that you start out by simulating your model for 1000 cycles without any forcing enabled. Hence, the last cycle of the simulation data so far will then be cycle number 1000. Then you force one of the input signals to another value. The value of the input signal itself and all the signals that depend on it will immediately be updated in cycle number 1000, while cycles 0 through 999 will stay unaffected. Finally, you run the simulation for another 1000 cycles, stopping at cycle number 2000. Now all the cycles 1000 through 2000 are affected by the forcing. If you now disable the forcing again, cycle number 2000 will immediately be updated to reflect that no signal is forced, while cycles 1000 through 1999 are still affected by the forcing.
Forcing is enabled by clicking in the Value or the Force column. Depending on the type of the signal, the following happens:
Forcing is disabled by right-clicking the signal and selecting Unforce in the context menu that appears.