Sinking into a ball pit is traditionally read as childhood joy and letting go of adult seriousness. It can also stand for being overwhelmed by many small things at once.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.