Citation Count: This is a simple summation of the number of citations for each article. You can find this number within Web of Science, Dimensions, Google Scholar
Relative Citation Ratio (RCR): Used by the National Institutes of Health, the RCR is a field-normalized metric. "The RCR is calculated by dividing the number of citations a paper received by the average number of citations an article usually receives in that field. That number is then benchmarked against the median RCR for all NIH-funded papers."