If someone is unfit, they're not qualified. You're unfit for a job as an ice cream truck driver if your only driving experience has been steering a bumper car at an amusement park.
Farmland can be described as unfit for planting if it's been leached of nutrients, and a babysitter might be accused of being unfit if he gets distracted and loses track of his charges on the playground. In either case, the unfit thing or person doesn't meet the necessary requirements or standards. Another way to be unfit is to be out of shape or in poor physical condition. Fit can mean both "suitable" and "in good shape."