it simply means for all x and for all y in the relation R , R(x,y) is same as R(y,x)
you can also say that R is a symmetric relation .
consider an example of group theory where elements of the set (1,3,5) on relation R is defined as x is inverse of y .
now according to the question , if 1 is inverse of 3 , then 3 will be inverse of 1 and so on..
hope it helps .