The particle に here is a clue that this word is an adverb. An adverb describes actions, so what's a normal action? It's one you're always, or constantly, doing. That's what makes it normal!

The reading is the kun'yomi reading, which you didn't learn. In order to learn it, think of this:

I always tune (つね) my violin, but today I didn't tune it. So, now I sound terrible. Imagine how terrible my untuned violin sounds, compared to how it always sounds.