When you see a singer with one hand up to their ear, they are actually using a DIY monitor system, or foldback system (listen to how they actually sound). You can hear what’s going on around you, but your ears are designed to block out a lot of the sound that you actually make, as the sound you make is usually formed inside your head. This causes you hear yourself differently to how people really hear you. This is why, when people hear a sound recording of themselves, you always get a similar reaction – ‘do I really sound like that?’
By cupping your hand from your mouth to your ear and singing or speaking, you can hear yourself much clearer and how you actually sound. This means you can check that you are singing in tune with one ear, whilst listening to the other voices or instruments with the other ear.
Some singers will also place a finger in their ear, this enables them to hear their resonance of the sound being made inside their head. It can help with their tone and resonance.