Active mass implies that amount of mass which is taking part in a reaction, that it is the effective concentration of a substance.
It's other names are reactive mass or chemical activity.
Units: The units of activity are nominal, rather than real, because chemical activity is formally defined as the ratio of the actual chemical activity of a substance to its chemical activity under some defined standard conditions, and ratios have no units because the units divide out. Or simply it's the ratio of concentration at required conditions to standard condition.The difference between activity and other measures of composition arises because molecules in non-ideal gases or solutions interact with each other, either to attract or to repel each other. The activity of an ion is particularly influenced by its surroundings.
For example : with the hydrogen ion, we use pH = − log10 [H+ ] where [H+ ] is the concentration, i.e. the number of particles (in moles) divided by the volume (in litres).
Once you get a lot of these particles, they bang into each other and take up room (think of billiard balls on a pool table or a room full of tennis balls flying around).
In these circumstances we talk about ‘activity’, which is a way to correct for this. Basically it is what the concentration or amount appears to be if we were using the simplified laws (the effective concentration). For the hydrogen ion, it is also what the concentration appears to be to physiological systems. In other words it uses a correction factor for the simplified laws.
Also Wikipedia says:
In a solution of potassium hydrogen iodate [KH(IO3)2] at 0.02 M the activity is 40% lower than the calculated hydrogen ion concentration, resulting in a much higher pH than expected