This usually refers to the level of background interference present to receiving clear sound.

Any additional sound outside of the actual sound we want to hear becomes a noise or sound “environment” (e.g. other voices or conversation in a restaurant, traffic, machinery in the background, tinnitus).

The versatility of modern hearing aids is such that they can now detect changes in background noise, adjust to each “environment”, so that you can still understand (for instance) the person and conversation you want to hear.