Your fighter's Ki Gauge Meter can be found at the bottom of your side of the screen, being measured with a number of bars. You can have anywhere from zero to seven bars at any given Point in the battle, each allowing you to do something special with your fighter on the field of play (and being kept no matter how many times you switch between your fighters). Vanish Attacks and Special Attacks require an entire Ki Bar to be used, while Super Moves require anywhere form one to seven Ki Bars depending on the character and the Super Move you are trying to use.
You can build your Ki Gauge Meter in a number of ways: moving towards your opponent, hitting an opponent and being Hit by an opponent. You can even gain more bars in your Ki Gauge by holding down the Light Attack and Special Attack Buttons at the same time to charge. Specific characters also have moves that will further boost their Ki Gauge Meters, including Android 18's Barrier Attack. But be warned: if you spend any of your Ki Gauge Meters during a combo and keep hitting the opponent afterward it is complete, you will actually continue to lose Ki.