You generally see this phrase when you're dealing with anything that can be planted in late winter to early spring, such as potatoes, onions, leeks, cool-season greens, and carrots. It means that the soil is neither frozen nor too wet (working in very wet soils messes up soil structure and results in bricks if you have clay soil.)
So, how can you tell if your soil is ready to work? Simple. Dig down three to four inches, and try to make a ball with this soil. If it doesn't make a ball, it's dry enough to work. If it does make a ball, try dropping it from about waist-high. Does it break apart into small pieces when it hits the ground? Then it's ready to work. If it doesn't break apart, or just kind of splits in half, it's still too wet to work. Wait a couple of days and try the ball and drop test again.