A loop plays, then through a low-pass or high-pass filter, and you find the cutoff frequency. Filters are everywhere in production, from cleaning low end to building transitions, and knowing where a cutoff sits by ear saves a lot of guessing.
At level 1 the cutoff sits between 250 Hz and 5 kHz, with about an octave of margin. Each level widens the range and narrows the margin, until the cutoff can land anywhere from 50 Hz to 16 kHz and you need to be within a small fraction of an octave.
Listen to what disappears from the bottom: first the sub, then the weight of the kick and bass, then the body of the midrange. The last thing you still hear clearly at full weight tells you roughly where the cutoff is.
Yes. EQ trains you to hear a bump or a dip; filters train you to hear where the spectrum simply stops, which is a different and very common decision in a mix.