That's the thread pitch, i.e. the distance from the peak of one turn to the peak of the next. The best way to measure it without a gauge is to count 10 turns and measure how much length they take up, then divide by 10. For this sort of value you can measure the length of a single turn with callipers but that's a less reliable method.
It looks like there are M12x1.0, 1.25, 1.5 and 1.75 through axles so you do need to be accurate. The way I've written it in the last sentence is the more common notation in engineering.
Metric threads have a pitch, which is a length, while imperial threads usually have the thread separation given in turns per inch (tpi). You may be more used to the latter.