The macro is very simple if you know any programming languages at all. If you were going to write this in a text programming language, it would look something like this:
NowControl = SECONDS(); // get the current time in seconds since boot.
if LastControl == "" then
LastControl = 0; // if the variable LastControl is undefined, set it to 0
if LastControl > NowControl - 1.0 then // if the last tap of the Control key happened less than 1.0 seconds ago
LastControl = NowControl; // remember the time this tap of the Control key
Keyboard Maestro is simply a visual programming language. Ironically, I'd not actually in favor of visual programming languages for most purposes because, as seen above, the text version is not much harder to understand. The reason it works well in Keyboard Maestro is that the programming aspect is an addition, allowing you to stretch and amplify the power provided by the actions, and the actions in turn get a lot of power from their visual editing that would be much more challenging to do in a textual programming language.
So to answer your specific question:
The variable LastControl remembers the time at which you last tapped the control key (the last time this macro was run). If you made another version of this macro (eg Double Tap Command) you would need to use a different variable.
The variable NowControl is a temporary variable that holds the current time. It's no longer needed as you could use the TRIGGERTIME() function instead - if LastControl > TRIGGERTTIME() - 1.0.
The "- 1.0" is "1 second ago". It is the allowable amount of time for it to be considered a double tap of the control key as opposed to pressing it, waiting and then pressing it again.