Spearman Rank Correlation tests the association between two ranked variables, or one ranked variable and one measurement variable.
Ranked variables, also known as ordinal variables, are those that can be ordered from smallest to largest, even if their exact values are unknown (e.g.: monkeys of a given species ranked in function of their dominance).
Spearman Rank Correlation tests whether the two variables co-vary, that is, for example, if when one variable increases, the other decreases. If one of the variables is a measurement variable, it first needs to be converted to ranks before using Spearman Rank Correlation.
You can read more about Spearman Rank Correlation here.
Source: ‘Handbook of Biological Statistics’ by John H. McDonald