In this Bach musical monogram, Bach spells his name musically. The circle in the middle is a note. On the left, we see a G-clef (aka the treble clef), its third line up represents B. This staff line passes through the circle to indicate that this clef gives us a B note. Note that this line also has flat sign on it next to the clef. In Germany, b-flat was represented by a standard B or b-rotunda.
Moving clockwise, the next clef we encounter is the tenor clef. When you see this type of clef sign, know that the C-line passes through the skinny part in the middle of the symbol. The staff line that passes through the circle is the A-line. So, that clef is giving us the A note. The next clef over is the same clef symbol but notice that it is located on a different place of the staff. On the aforementioned tenor clef, the C-line was located 4 lines up the staff but here the C-line is in the middle of the staff and that line is passing through the circle. That clef gives us the C note. On the bottom clef, we see another treble clef. The B-line passes through the circle and that line has a natural symbol on it. B-natural in Germany back then was written as a "hard B" or b-dura which we would recognize as an upper case H. So, in order, the clefs give us B-A-C-H.