Yes
Back pressure is the measurement of the standing wave created by the exhaust leaving the end of the muffler
the 2" of water is the measurement on the vacuum gauge where the 40 ( no units ) was taken.
It is an emission requirement in Europe.
From it you determine how well the engine scavenges the exhaust out of an engine and how much unburned fuel will be drawn out with the exhaust.
All part of the very black art of exhaust tuning.
And because back pressure is a standing wave it is frequency dependent , the frequency is the number of exhaust pulses so at 3600 rpm on a single it is 1,800 and on a twin it is 3,600.
Too much back pressure & the exhaust won't come out and not enough back pressure will cause too much of the incoming fuel being sucked out the exhaust, thus the exhaust will have too much unburned fuel in it
It is the same thing as carburettor reversion where the carb blows back into the air filter rather than going into the cylinder
People see the carb spitting back and think it is caused by the exhaust in the cylinder blowing back through the inlet during valve overlap
But running engines with no overlap & seeing the same thing happen put pays to that.
A few decades ago the idea was to feed carbs from a Pheleum chamber so it was sucking still air
Before that it was ram tubes .
Critical with 2 stroke design but not quite so important with 4 strokes unless you are trying to extract that 11th tenth of Hp
