What do you think notation a ∩ b means

It means set intersection

The intersection of two sets is the overlapping or shared common elements in both sets.

For example, lets say we had

a = {1,2,7,9,14,3}

b = {14,6,1,3,5}

The common elements in both sets would be {1,14,3} which is in the set

If you drew out a Venn Diagram, then you should see that 1,14, and 3 are in the overlapped region. The other numbers are outside the overlap region but inside their respective circles.