What is this text above the staff for, and what is it called?
It's called a Performance Mark, Dynamic Mark, or Directive, meaning an instruction, generally from the composer, regarding exactly how a particular part should be played: Loud, Soft, Slow, Fast etc.
Boldly! is modern English. However, in classical music and more traditional scores and sheets, Italian terms are generally used (without exclamation points), such as these:
Further down we find this:
risoluto risoluto bold; strong
I've always known that as an 'expression marking'. It's the composer's indication to the performer of the style and often speed of the piece.