Well, SD obviously means that you're shooting and it's saving on to the SD card or that one is in fact, inserted in the camera and ready to record data.
The music symbol means that audio from focus confirmation is enabled. (the beep that happens as focus aquires)
The two people mean that (and this is my best guess here without consulting the manual) that ADL is not turned on.
Did you look up the info in your manual yet? That's what it's there for...
Look in the online reference manual under The Information Display. If you're using a D5100, it's on page 5. It shows all the symbols and their meaning.