Left and right channels
The Left button sends sound to the left channel only and Right to the right channel only. Use it to check that earbuds are in the right ears and that speaker cables are not swapped. If one side is quiet or silent, check the cable, the jack and your system's balance setting. With mono audio turned on in accessibility settings, both sides sound the same.
Test tones and hearing range
Human hearing is commonly given as about 20 Hz to 20 kHz, and the ability to hear high frequencies usually declines with age. The sweep rises from low to high over 20 seconds; stop it when the tone disappears to see that frequency.
The result also depends heavily on whether your speakers or headphones can reproduce that frequency. Small laptop speakers often cannot play deep bass. This is not a medical hearing test, so see a professional if you are worried about your hearing. Turning the volume way up because you cannot hear a tone can strain both your ears and your speakers.
Polarity (phase) check
If the + and - wires of one speaker are swapped, the two speakers move in opposite directions, bass gets weaker and the sound no longer focuses in the middle. In phase sends the same signal to both sides; Out of phase inverts one side. With correct wiring, In phase sounds clearer and centered.