Q: What is citation score?
As far as we know there is nothing like citation score. There is the CiteScore, which is a metric to help you track journal performance). Basically, it's a metric like impact factor, SNIP (Source Normalized Impact per Paper), or SJR (SCImago Journal Rank). CiteScore metrics include 8 indicators that monitor the citation impact of a journal by determining the number and percentage of citations and documents in any journal. These reflect on the impact of the journal.
You can find more details about CiteScore here.