The ∩ sign stands for set intersection, and |X| stands for the cardinality of the set X – basically the number of elements in the set.
This coefficient measures the similarity between sets X and Y. If the two sets are identical (i.e. they contain the same elements), the coefficient is equal to 1.0, while if X and Y have no elements in common, it is equal to 0.0. Otherwise it is somewhere in between.