That is the member's "Feedback Score". It is calculated from the number of positive feedback ratings s/he has gotten since joining (as opposed to the feedback percentage which only goes back 12 months), adjusted to not count more than one positive rating per seller per week, minus the number of negative ratings (also adjusted for multiples from a seller per week) but since sellers have been blocked from leaving negatives to buyers for at least a decade that only applies to sellers and buyers who got negatives before then.
It is an approximate indication of the frequency of trading under that account but does not include transactions where the other side didn't leave feedback or left a neutral and for multiple transactions with the same other user per week, and negatives get subtracted instead of added for the total.
If the user ID of the member is fully displayed (like on these forums) you can click on the feedback score to get to the member's Feedback Profile which contains a wealth of information about the user's activity (especially the Feedback Left for Others which sellers who have the time and inclination to do so check--the one you left for postage due could have been edited down to the essential facts and may cause you problems in the future--and the number of bid retractions in the past 12 months on the Feedback as a Buyer tab). In a Bid History clicking on the score brings up some statistics on the bidder's bidding, mostly over the past 30 days. Some users have opted to make their Feedback profile--"(Private)" displayed instead of the score--which restricts some information on their Profile; that is generally not a good idea since you can't sell with Private Feedback and a seller seeing that will assume the worst in what the prospective buyer is trying to hide and it does not block any of the statistics from clicking on it in the Bid History.
