Identifying key concepts means determining which words in a research topic will be best when searching for information sources.
Let's look at a research topic and determine the key concepts to use (and also the words you won't use) when searching:
Topic: I am interested in finding out how electrical stimulation can help improve overall athlete performance.
Key Concepts: "electrical stimulation" "athlete" "performance"
What words were left out and why: "Improve" "Help" "Overall" -- If a study has been conducted on the links between athlete performance and e-stim, then a scholarly research study will discuss the outcomes of the experiment, whether the outcomes are negative or positive. The key concepts of the topic above deal primarily with e-stim and athletes, so use those concepts as keywords to help you get started.