Generally, in any table or relation, an attribute or a set of attributes is used to determine the value of rest of the attributes in the table or relation. This is possible as the rest of the attributes are the functions of the key column. This situation is called functional dependency and it is possible when there exists one and only value for each non-key attributes that correspond to the key attribute.
Consider an employee table, where employeejd is the key and rest of the attributes, such as name of the employee, can be determined by employeejd as they are functionally dependent on the employeejd. For example, if there is an employee whose name is Ram with employeejd 1001, then the name corresponding to the employeejd 1001 will always be Ram only. No other name can be associated with this employeeid.