granny gear n. the lowest gear available on a bike, which only a grandmother would need to use; designed for steep uphill climbing, but extremely easy to pedal in on flat ground.
From the Dictionary of Mountain Bike Slang
While useful, there are a couple of things to keep in mind when thinking about giving up and using your granny gear. The first, is that once you're in granny gear there's no turning back. By this I mean don't start using it at the very start of a climb; always keep you granny gear in reserve. So unless the going gets so tough that you're about to stop, don't use it.
Secondly, keep in mind that using your granny gear puts a lot of torque on the drivetrain. Keep your pedalling motion smooth (no biopacing!) so strain on your bike is minimized. Don't combine granny gear with standing up, or you'll set yourself up for a broken chain or spoke.
And thirdly, be careful not to wheelie and fall over backwards. With all the torque you're generating the front wheel will probably be trying to get airborne, and it won't take much to raise it up. Falling on your back isn't fun!