I came across this notation in a guitar solo collection book, but I don't know what it means. It's a bracket spanning a chord (The Asus4, A/C# and G chords in the image below.)
My first guess is that it means to form a barre chord, but the same book uses the notation "C.3" or "C.5" for barre chords.
EDIT:
The same piece also contains wavy lines.
The piece is the "Opening Theme" from Final Fantasy. The piece comes from this book, Final Fantasy Guitar Solo Collection. The book is Japanese if that matters.
I had a closer look at the other pieces in the same book, and here are some more images






