Data island is nothing but a associated group of tables. Say your data model has 10 tables, out of which 7 are interconnected to form a group1 and three are interconnected to form group2; group1 and group2 are nowhere linked. In such circustances, it would be said that your data model has two data islands group1 and group2.
This expression will check whether 'Revenue' has been selected (green) in the list box and enable the expression column if so. Note that there is no direct connection between table Expressions and anything else in this data model.