ATMEGA328

Basically bootloader is a little piece of code that allows you to program the flash memory of the Arduino’s atmega328p via serial or USB instead of using an ICSP programmer.

If you are making a project based on Arduino UNO & you have made a customized PCB then you will be requiring bootloaded Atmega328 microcontroller. This is a ATmega328 in DIP package, pre-loaded with the Arduino (16MHz) Bootloader. This will allow you to use Arduino code in your custom embedded project without having to use an actual Arduino board. To get this chip working with Arduino IDE, you will need an external 16MHz crystal or resonator, a 5V supply, and a serial connection. You should select the Arduino UNO board within the Arduino IDE.