Your mass is the same no matter where you are or what you are doing. To lose mass you have to get rid of some of the stuff you are made of. To gain mass you have to add to the stuff your body is made of. Mass is often measured in pounds or kilograms.
Acceleration is a change in speed or direction of motion.
To understand acceleration, imagine that you going to give a first grader a ride in their red wagon. You decide to give the kid a ride on a long block. You start off very slow, moving one step a second. You steadily go faster so that when you pass the first light post your speed is 2 steps a second. Because your speed has been steadily increasing you have experienced constant acceleration.