Experience is a mechanic used to gain training points. When in the normal minecraft world, punching with available ki will make a special sound, When this sound goes off, it means that you used your DBC strength to hit and you gained one experience point. Every ten of these experience points gives the player one training point and ki attacks give the player a random undisclosed amount of experience.
Inside the Hyperbolic Time Chamber or on King Kai's planet, this mechanic is a bit different. In either of those places you start training where it takes a small amount of experience to gain one training point, but as you gain experience you will need more for the next point. Inside one of these training areas, it will tell you how to gain experience.