Through Axle Wheels thread P1.0 or P1.5 I'm looking to use some Pinheads on my bike. The front wheel is a through axle wheel. From Pinhead's product website they have an option for P1.0 or P1.5 thread - I'm looking for a M12 124mm long bolt.

I'm unsure what P1.0/P1.5 signifies on the thread, and also unsure on how to measure it.

Can anyone explain what the P-value signifies? And how I would measure it?
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. 

Further reading:

http://bikeandskiblog.com/wp/2016/01/12/mtb-thru-axles-who-knew-there-were-so-many-standards/