What is the noise-floor-threshold 
you can set what the card deems an “acceptable” signal… like say you dont want users connecting with a signal less than -85dBm. You’d set your noise-floor-thres to -85 and then users would have to obtain a signal better than that.
At least that’s my understanding.
…which I share. That’s exactly what this setting does - telling the card (or RouterOS) to ignore wireless signals which are below the value configured here.