Dogs that are putting less weight on a limb are doing so because it hurts. The common reasons for lameness are injuries or wounds to the feet/pads, strains or sprains, ligament damage (most commonly this is the cruciate ligament in the knee, similar to people). Less common but more severe would be joint or bone injuries like fractures (broken bones).
Mild lameness = dog is still using the leg and putting some weight on it. Check paw pads, between toes, and toe nails. Treatment for mild lameness is strict rest, an ice/cold pack if you can identify the site of pain, and consider an anti-inflammatory pain medication if available.
Moderate to severe lameness = toe touching or non-weight bearing on the limb.