The simplest explanation would be by implementing Runnable we can assign the same object to multiple threads and each Thread shares the same object states and behavior.
For example, suppose there are two threads, thread1 puts an integer in an array and thread2 takes integers from the array when the array is filled up. Notice that in order for thread2 to work it needs to know the state of array, whether thread1 has filled it up or not.
Implementing Runnable lets you to have this flexibility to share the object whereas extends Thread makes you to create new objects for each threads therefore any update that is done by thread1 is lost to thread2.
