Read the first entry for crab bucket and you will find what is meant here.
This is usually somebody that is close to you that gives it; they exploit your trust. It is when somebody tries giving you advice that, either directly or indirectly, is meant to influence you to make a decision that ultimately affects you negatively. This is done by telling you that your job that affords you a better lifestyle than they have/had at that age is a bad idea, trying to convince you that you are stupid when in reality your intelligence and potential are much higher than theirs, telling you that the potential mate you can obtain that is better than they can get is a long list of negative things, etc.
Unfortunately, this tends to work, because most people would never suspect that somebody close to them would give them such selfish and malevolent advice. Hell, most people don't even know what "crab mentality" really is. Another unfortunate thing, is that even if you know beyond any reasonable doubt what the ulterior motive is, especially if people victimize themselves when confronted with something like this, good luck trying to convince anybody of it.
Please use even a bit of critical thinking when it comes to the things that people say to influence you, even if it seems like it is in your best interest; ESPECIALLY if they try persistently. :)
My mom and dad think it is a bad idea for me to be a bartender. Apparently, they think that slaving away twice as many hours a week earning minimum wage is going to allow me to get a higher GPA. I think it is just BS, and more crab advice from my parents, who suck at life.