What does it mean if my dogs limping?

My dog is named Braxton. He's a Lab! Hes loves taking walks and hes playful. My dad thinks hes getting old. I dont think so. Hes 120 pounds a little over weight and we havent took him to the vet in 3 years because its to expensive. 6.5 years. I love him to death and i wnat to help him PLEASE HELP thanks, anna