Good luck Megan!!
In earlier times, actors wished one another “may you break your leg”, in the hope that the performance would be so successful that the performer would be called forth to take a bow — to bend his knee.
The curtains on either side of a stage were called the legs, so that to pass through the legs was to make it out on to the stage ready to give a good performance, or perhaps expressing the hope that you will need to pass through them at the end of the show to take a curtain call, implying your performance had been good.
The saying really refers to getting one’s big break, that the performance will be good enough to ensure success in one’s career.