I actually had to Google the word yesterday.
It's a relationship of two things changing and affecting each other, and I think in a proportional or ratio kind of way. As for what it does in the code, I'm not 100% sure, but it has to do with the relationship between distance and the buzzer/LED activating or not. I have to go soon so I don't have time to test it right now, but I'm going to raise and lower the number and see what happens.