If you're looking for specifics than you'll have to accept that they'll get pretty technical, but we can create an intuitive example that works, even if we wouldn't do it this way. Let's say I focused a laser on a mirror that bounced back to a photosensor. The light travels on a V path basically, emitting, hitting the mirror, then bouncing back to a photo sensor right next to the laser, the mirror is just very slightly tilted. When the light hits that photosensor it sends two signals, one to the laser, telling it to turn on again, and another to a counter that adds 1 every time it gets a little electric pulse from the photosensor.

Now, this is a simple clock, each 'tick' occurs when the photosensor is struck. How long that takes depends on the distance to and form that mirror. If that mirror is 150 kilometers away then it travels 300,000 meters, which is 1/1000th of the distance light covers in a second... we now have a clock that ticks every one-thousandth of a second, or is accurate to the millisecond range. If I drop that mirrors distance to 150 meters, it will tick every 1 millionth of a second, or microsecond range, if I drop that range to .15 meters, basically a one foot path for our laser, we've got a 'tick' of a billionth of a second or a nano-second. Our clock, the little counter that receives pulses form the photosensor, can be wired up to something else as well, whatever you're measuring is set to flip a switch when something occurs, like you have a big hourglass whose bottom plate is actually a scale with marks for every gram, and every time it gets another gram it sends a signal to our counter, which takes that and the time and stamps it. You get "150 g at 300 ns, 151 g at 302 ns, 152 g at 303 ns" etc and can say mass accumulates at approximately half a gram a nanosecond... which would be about 500 tons a second so one big hourglass but nevermind that.

Now, the reason this gets technical is that we wouldn't do it quite that way, and you get a host of technical problems such as how long does it take for the laser to fire after it gets the signal, how much does the air slow the laser down by, how long does it take the photosensor to absorb light and dispatch a charge to the laser and counter, how long does that signal take to travel to each, are they uniform in duration and to what degree, etc... and those are the factors that limit your accuracy and thus your minimum measurement, and that gets very technical and there's a bunch of different processes one can use to do this.

Hope that helped.