Okay, I know they have these things called clippers that come in sizes 1 - 4.
I have no idea what these numbers mean. This is one of those things that probably everyone knows except me…but since I’ve never used clippers, its just not something I have any experience with.
Actually, the blades come in more sizes than that.
I didn’t know either, untill I looked it up. I’m pretty handy with Google 
For Oster clippers, the different blade sizes cut hair to the following lengths.
Blade Size Hair Length
00000 1/125" 0.2mm
0000 1/100" 0.25mm
000 1/50" 0.5mm
0A 3/64" 1.2mm
1 3/32" 2.4mm
1A 1/8" 3.2mm
1 1/2 5/32" 4.0mm
2 1/4" 6.3mm
3 1/2 3/8" 9.5mm
3.75 1/2" 13mm
The blade numbers and lengths are probably different for different brands of clippers.
Does that chart indicate how much hair is REMOVED (blade 2 removes 6.3mm) or how much hair REMAINS (blade 2 will leave behind 6.3mm)?
I’d guess left behind - how could it determine how much hair is removed? You might not have the clippers flush against the skull, or whatever it is you are clipping.
Test on your dog or SO - you’ll probably find that the measurement is the minimum length of hair left on the head, or arm or back or whatever.
It’s how much hair will remain. I’d imagine that it’d be pretty difficult to make clippers that only cut off a specific amount of hair.
I’m a shaved head type of guy, and from the different barbers I’ve talked to, guard depth varies. One guy said “Are you sure you want a 1”? and showed it to me. His explaination is that some other companies will call a thicker guard a “1”. I use a Wahl with no guard, and no I’m not a “skinhead”. My 1 leaves pretty close to 1/8 of an inch of hair, and with no guard I get a bit less than 1/16th. So, to answer your question, guard numbers are relative and have no standardized length associated with every brand. Remember to start with a high number and work your way down as needed, you can always remove hair, but only time will grow it back.