What does this symbol in the piano score mean and where is it played?

This symbol is called "G clef" and "treble clef". However, the treble clef has nothing to do with which hand to play. The line where the treble clef is located is not necessarily played by the right hand. So, which hand do you play with? For two lines of piano music, as long as there is no special explanation, you can play it with your right hand up and your left hand down, regardless of the clef at the beginning of each line. So the music attached to you is also placed with your right hand on the top line and your left hand on the bottom line. Let's talk about another symbol you mentioned. Symbols like "B" are called "flat symbols". Where the flat sign is, it means which note is flat. For example, in the attached music score, the two flat notes are marked with 7 and 3 respectively, so these two flat notes mean 7 and 3, not just 7. The flat note written at the beginning of each line of the score is called key signature, which means that all 7' s and 3' s in this line should be flat notes to 7' s and 3' s. This key signature's explanation of 7' s and 3' s shows that if the music is a western major or minor, it is a B-flat major or a G-flat minor. If you don't understand anything, you can keep asking me. :)