A relation is said to be in Functional dependency when one attribute uniquely defines another attribute.
For Example, R is a Relation, X and Y are two attributes. T1 and T2 are two tuples.
T1[X]=T2[X] and T1[Y]=T2[Y] means the value of component X uniquely define the value of component Y.
Also, X->Y means Y is functionally dependent on X.