What is this big lever on the right?

[CobyRupert]

Wait until you find mud/ice or dragging something stubborn around like a plow.

Yep, differential lock is good when one tire is spinning in the mud or ice, and the other tire is just sitting there not applying enough torque to propel tractor forward.

The other trick is to just hit the brake for the spinning wheel so more (enough) torque goes to the wheel with traction. No diff. lock required.