How to rate my level of proficiency in statement for graduate application
This sounds to me like a good place to follow the dictum of "show, don't tell." Giving a numerical or qualitative judgement will be useless, because they can't tell how your rating scale relates to their own.  What you can do meaningfully, however, is talk about what you have actually done which demonstrates facility with the concepts by putting them to use in classes, projects, or real work.

For example, if one of the concepts was "algorithms," then you might talk about a piece of software that you wrote that required understanding and applying complicated algorithms, or an undergraduate research project in which you created and analyzed an algorithm, or something of the like.  This will then give the evidence that the reader needs to assess you accurately, however they calibrate their scale.

If you don't have strong examples like that, then just say whatever the truth is about what you've done with the concept, e.g., "I got an A in my algorithms class."  It's not very satisfying, but in a case like that you really don't have much facility with the concept (peopler generally understand a concept much more deeply once they have applied it outside of the classroom).  Better a less than satisfactory truth than trying to guess what somebody who you've never met would think differentiates "pretty good" from "expert."