Try this: take a piece of string or dental floss and wrap it around your lead screw such that it never leaves the groove it starts in. Here’s an example with the lift screw on my table saw.*
As you can see, the thread on my lead screw causes the string to come back around to the groove right next to the one I started in. In other words, the groove in my lead screw is one continuous groove. It’s a “one start” lead screw.
On a four start lead screw there are four grooves, so if I wrap a string around a four start, the string will come back around with three intervening grooves. Because of this, every 360° of the screw causes the nut to move four thread pitches.
I’m not sure if I explained that well. Did that make sense?
*Don’t ask me why my table saw is torn down so far that I can photograph my lead screw. It’s a long story in which Mattias Wandel got me curious about something and “made me” try an ill-advised experiment. Replacement parts are on order.