Things worth pointing it at
Whistle — you'll see one clean spike, because a whistle is nearly a sine wave. Then say "ahhh" and watch a picket fence appear: your vocal pitch plus its harmonics, evenly spaced. Play a tone from the generator on another device and check the spike lands where it should — a two-device honesty test. Household mysteries work too: that fridge hum will show its exact Hz, and now you can name your enemy.
The axis is logarithmic, an octave per equal distance, because that's how ears count. The tall stuff below 200 Hz in any room is normal — rooms rumble.