M10 is the nominal bolt diameter in the metric system, so its about 10mm (but always slightly less)
1.5 is the pitch, or distance between each thread, in mm. Sometimes specified as "coarse" or "fine". For an M10, 1.5 is coarse, 1.25 or 1 is fine.
30 means it is 30mm long not including the head.
Hope I've got that right
As an aside - the pitch tells you how far a bolt will move axially when you make 1 full rotation. I.e. a 1.5mm pitch bolt is pulled 1.5mm further into a hole when you turn it 360 degrees... This is useful to know for making tie-rod adjustments and that sort of thing.