What is the use of 1k resistor shown in the picture and why it is used? Please check Arduino Uno Schematics for more info
Its a pulldown resistor!

Comment there says "USB Boot Enable" so safe assumption that it's an digital pin.

They're handy because they mean if there is no input to whether that pin is headed, the voltage at the pin will be 0 instead of floating.

The value doesn't matter too much, "When in doubt 10k it out" is a pretty well known saying around electronics people, meaning just use a 10k Ohm resistor and you'll be fine.