I’ve done something similar, but for radio stations. Several times. Apparently once you get put on the list, they’re more likely to keep coming back to you. Over the years I was also approached to take part in various product panels too, probably as a result of being on the list of reliable participants.
The radio panel just consists of, as @SEKA explained, a booklet divided up into half-hour segments where you tracked your radio-listening habits for one week, then mailed it back to them.
(I should add that I take pride in always screwing with marketing data. When I do the radio panels, for example, I develop a burning interest in listening daily to amateur campus radio stations. And when I do product panels, my preference in something like diet cola runs to the bitingly bitter with a strong chemical aftertaste.)