What does it mean that a symbol in the staff that looks like a sharp sign but is not a sharp sign, that is, an upright inverted L-shape, is closed at the top and bottom?

The symbol you are talking about may be the Shuang Sheng in music, which usually means that a tone rises by two and a half tones. The shape of this symbol is an inverted upright L-shape, and the middle is closed. In some musical instruments and music theories, this symbol is used to indicate that a note should be a semitone higher than the normal note.

I hope this answer will help you! If you have any questions about music or score, please feel free to ask.