it means leaving the implementation to the implementer, 
i.e. interface. you leave the implementation of 
functionality to the class who is implementing this 
interface

In a realization relationship, one entity (normally an 
interface) defines a set of functionalities as a contract 
and the other entity (normally a class) "realizes" the 
contract by implementing the functionality defined in the 
contract.

It is similar to generalization.It indicates that one class 
implements behavior specified by another.

Realization denotes implementing an interface.Arrow with 
dotted line 
Generalization denotes extending a class.Arrow with solid 
line.

I just want to add that realization is not only for 
interfaces or class it is also for use cases and use case 
realizations is to design its inner diagrams like sequence 
diagram …etc.