Odin mode, also called Download mode, allows Samsung phones to download and flash the ROMS and Kernels. It is an exceptional mode that comes pre-loaded with the phone. It enables you to perform tasks on your Samsung device. The tasks can't be done if your phone is not in Odin mode; you have to reboot your phone into Odin mode first.
To enter the Odin mode, press and hold Volume Down, Power, and Home buttons simultaneously.