The Semaphore actions (Lock, Unlock and Reset) allow you to prevent (a) multiple triggers of the same macro from executing simultaneously, and/or (b) multiple macros from executing simultaneously.
When you trigger a macro, the Keyboard Maestro Engine takes a copy of it and starts executing it. If you trigger another macro before the first one finishes, that will start executing as well - both will be executing more or less simultaneously.
Although it is rare to have multiple macros triggered at the same time, the normal behaviour in Keyboard Maestro is that all triggered macros run simultaneously, with actions from each independently triggered macro running at the same time.
Most macros are triggered explicitly manually, and typically the user waits for them to complete before they trigger another macro. So macros triggered like this will rarely run at the same time.
But there is another class of macros that operate in the background, most especially time triggered macros. Macros that are triggered independent of the UI and the user's awareness. Typically these will be written so as not to impact the user since the user won't be expecting them to run. So they might be things like backup scripts, or automatic download scripts, or the like. Things that happen in the background, or that wait for the user to be idle before doing stuff in the foreground. Often these sorts of macros run for a significant amount of time.
Occasionally there are cases where multiple macros can be triggered, or the same macro triggered again, before the other macros complete. This can sometimes cause a problem, which needs to be prevented.
The Semaphore actions require a Semaphore Name, which uniquely identify each Semaphore Lock. Use a different Semaphore Name to identify different (independent) Semaphore Locks.
In order to prevent simultaneous executions (instances) of one or more macros that have Semaphore Locks with the same Semaphore Name, do the following:
Semaphore Lock can be balanced by a Semaphore Unlock, but a Semaphore is implicitly unlocked when the macro that locked it completes.
Semaphore Reset will forcibly unlock a semaphore and should generally not be used except in very unusual circumstances.