This is an originaly an automotive term..... manual transmission trucks used to come with a super low first gear (and some still do). They were for getting heavy loads moving, as the older trucks were seriously underpowered....no 400hp V8's like today, we're talking 100hp straight sixes.
So, you'd use granny to get the truck moving, then almost immediatly shift into second, as you couldn't get any speed in granny low (driving as fast as your granny walked, probably IS pretty much where the term came from).
The big GMC Topkick box truck we have at work has a six speed stick with granny low. When the truck is empty, we always start it out in second. In first, almost as soon as you've let the clutch out it's already time to shift into second...that diesel redlines at 2.5krpm!
Jeep guys LOVE these transmissions...leave it in first gear, low range, you can literally get out and walk faster than the Jeep is moving. And all the torque from that gearing lets them creep over damn near anything.