High-cardinality in SQL refers to highly differentiated data for a given column. (Although “relatively unique” is not grammatical, that is one way to say it.) When there are many different possible values for a given column, that is high-cardinality. In the context of monitoring, what is high-cardinality?
In monitoring like relational databases, cardinality refers to the number of elements in a set (consistent with the mathematical definition https://www.dictionary.com/browse/cardinality).