You can use it as a indicator of someone's behavior in this forum, if they don't have anything then it is unknown but you can always trust high ranking members because they will lost big time if they were to cheat you, positive could be an indicator that they are good in terms of transaction online but you always have to read the positive, neutral means that you have to watch out around that user or read the reason why they were put there, negative is the first deterrent of the forum to prevent scammers from getting more victims.
Please don't blindly trust anyone here, do your own research. The rank has little to no significance to their trustworthiness, there are plenty of examples of higher ranking members getting involved in scams.
The link to LoyceV's thread documents the proper usage of the trust system fairly well. Note that the Trusted/Untrusted feedback depends on how you structure your Trust list. Do try to modify your Trust List as you go along to include/exclude people whose judgement you find accurate/inaccurate.