In the daily life, some of the contacts are used so frequently that they are treated as the favorite by us. Understanding this philosophy, Android has the option to add the frequently called contacts in the favorite contact numbers.
It is easy to add contacts in the favorites but you may like to see the instructions, if need be!
Instructions
1: Tap the icon of ‘Phone Dialer’ app on the Home screen of our Android 5.1 Lollipop device to open up the ‘Phone Dialer’ app screen.
2: At the ‘Phone Dialer’ app screen, tap the settings interface button, placed at the top right corner of the contact image at the ‘SPEED DIAL’ to open up the settings interface.
3: At the contact interface, see the star button to add to favorite is not enabled. To check, just put the finger on star button and a tool tip will indicate its function, i.e. it is used to ‘Add to favorite’.
4: Tap the star button and see the star button is enabled now, i.e. this contact is added to the favorites.
5: Go back to the contact interface and see the star is added to the contact image.
6: To see the favorite contacts, tap the icon of the ‘Contacts’ on the Home screen of your Android 5.1 Lollipop device to open up the ‘Contacts’ app screen. 
7: At the ‘Contacts’ app screen, see the contact has been added in the favorite contacts.
That’s all!