A feedback loop is when cause and effect are related so that the effect influences the cause.
When the effect reinforces the cause it's a positive feedback loop (generally unstable), when the effect reduces the cause it's a negative feedback loop (generally seeks equilibrium).
In the case of the HPTA, the presence of increased amounts of testosterone lead to increased aromatization to estradiol, which in turn reduces the secretion of lutenizing hormone, which reduces the production of testosterone.
This is a negative feedback loop (I hope I didn't get it completely wrong) which helps keep levels of testosterone stable when everything is working as it should.