A locked door can symbolize opportunities that you have not worked towards yet, or opportunities that you haven't been able to achieve. Or, a goal you haven't been able to achieve.
Explanation:
A locked door. Locked doors are doors that need a key to be unlocked, right? So, if the opportunities/goals that you have are behind that door, you need the "key", which is the hard work and effort you put into achieving that goal.
When performing, you already have developed your technique and skill in practice sessions. Knowing that you have already mastered the technique and skill should help you to relax.
Through repetition of performances then comes the master of technique.