What alphabetic labels in phone dialer mean?

Question

I am writing a phone dialer for my apps in both Android and iPhone.

I realize that there are alphabetic labels below (or beside) numeric labels of keypad buttons in dialer layout of many calling apps.

example:

below '1' is blank

below '2' is 'abc'

below '3' is 'def'

...

Can any body explain why this happens?

and what does 'abc', 'def',... means in this case?

Updated:

Answer 1

if you are in numeric mode .. only numbers but if you are in other mode like text .. so you will use these characters ie: '2' is 'abc'

press 1st time ==> a press 2nd time ==> b press 3rd time ==> c

hold press on that button ==> 2 (number)

this is the mechanism generally.

=================update ...

:) you added this image later .. I didnt note it . well this layout is special and for quick contact search(names and numbers) ..ie if you have contact Sam(+111111), so press 726 , that will bring him directly or similar names or numbers. So those characters are keys map to help user to assign correct input... hope that will explain what you want .