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).
"[T]he number of individual values of a metric." This was taken from Splunk.com.
Time series data can be paired with labels, meta-data or dimensions. This combination increases cardinality from the atomic metric categories.
Creating artificially high cardinality can create large amounts of data to reconcile; some businesses demand high cardinality for certain analytical purposes.