Rotate that ring back and forth until your crosshairs are sharp and black and the target is in focus. But don't stare at the crosshairs. Adjust look, look away. Repeat until it's set the way you like. Your eyes can only maintain focus for so long. Moving them back and forth assures you have your crosshairs as sharp as possible.
The ocular's job is to bring the reticle into focus. The objective's job is to bring the image of interest into the same focal plane (or as close as possible) as the reticle's position. If all is adjusted and working correctly, the reticle is in sharp focus and the image of the target is also in focus because it resides in the same focal plane as the reticle.