Tap into = dial in, or access what it is inside you
your potential= the best of what you could do/make/become
'Tap into your potential' is an expression that means (to me) to dig deep (inside of yourself) to bring out the best in yourself so you can apply that to what you are doing/planning to do, etc.
Ex: A teacher might tell you to 'tap into your potential' if you made a C on the test and she knows you could have done better. A sports coach will tell you to tap into your potential when you are exhausted and only have 20 minutes left to win the game. You will tell your employer that you hope the company allows you to tap into your potential by offering you a variety of responsibilities, opportunities, etc.