"standard". It means that the volume of gas (in millions of...

"standard".

It means that the volume of gas (in millions of cubic feet) is being measured at Standard conditions (60F and one atmosphere pressure). The reason for its use is that a set amount of gas at wellhead pressures occupies much less volume (due to the high pressure) that at atmospheric pressure.

By quoting gas volumes at Standard conditions, comparisons between fields, flowrates etc (which may in reality be at very different pressures) can easily be made.