Taking Out the Garbage

Tonight, I heard about the metaphor of purifying the mind prior to meditation, particularly through repentance or some other practice, as something akin to 'taking out the garbage', or purifying the mind of garbage. While I think this metaphor is quite intriguing, I can't help but wonder, just what is meant by garbage, and what can this kind of practice really do for people? I am thinking that 'garbage' is really just the phenomena that comes from previous karma or past conditions, so there is no use in even branding it as garbage, per se. But more importantly, is there ever a point where person is truly 'purified' of anything, or might this sometimes be a kind of conceit, at times?
  I have sometimes had very genuine moments when I am prostrating to the Buddha, where I genuinely feel and know that all my vexations and thoughts are not worth attaching to, and I behold a space where I can truly accept the present moment with all the conditions that lead up to it. However, there is not much intention there on my part; it is just a sense of genuine and real refuge, knowing that I am simply not able to put down my thoughts using other thoughts. If on the other hand, I prostrate with strong expectations of a result, then I am in a subtle way imposing a goal of purity, and this actually affects the result by making me feel more tense in anticipation of a calm or 'pure' state. At times, I might even believe that I have reached that state, judging by the way my mind suddenly might feel calm or less burdened by the pressure of thoughts. However, where it becomes tricky, I guess, is when I turn this calm state into a 'marker' of my inner purity--something which many religions would consider a kind of false pride. It would be like praying to God and then thinking that God has answered my prayers in some way just because I feel a sense of relief after praying. When this subjective state becomes the goal of repentance, it can start to become more like a self-fulfilling narrative.
   On the other hand---when I prostrate without any expectation, I often find myself almost 'accidentally' having an insight into the value of prostration, without necessarily attaching additional meaning to that insight. For instance, I might realize how interconnected I am and in need of the vows and teachings of others, when I prostrate in respect to Buddhas and teachers--and this might result in an expansion of consciousness beyond the sense of self. But if I then make the 'state of purity' a goal, this in some ways diverges from interconnection by suggesting that there is a single 'self' that is purified by the practice. So I do think that these kinds of practices may need to be approached with care, humility and caution.