Originally Posted by CliffordK

A little more description of your bike would be helpful.

I usually ignore numbers and indicators on shifters (as most of my bikes don't have them).

However, in general:

Shift to small rear sprocket and large front chainring and you have the highest gearing; hardest to pedal/more force on the pedals, and longest distance per pedal rotation.

Shift to the large rear sprocket and small front chainring and you have the lowest gearing; easiest to pedal/least force on the pedals, and shortest distance per pedal rotation. Good for HILLS.

Other combinations will be somewhere in the middle.

"Cross chaining" is when you are shifted to the large front and large rear, or small front and small rear. Many drivetrains are able to tolerate it, but it is best not to be cross-chained.