Odin mode

Odin mode (or Download mode) is for Samsung Galaxy phones what the name means. You are a God and the phone is a slave. In Odin mode, the user is able to flash whatever wants to the internal flash memory inside the phone by connecting phone to the computer via USB cable. As Odin mode is simple program (in the phone), poor user may not even flash main (Android) system, kernel, recovery system, but this is possible to repartition internal flash and reflash Odin mode itself. If something goes wrong and the Odin mode is broken (erased, rewritten, corrupted), the phone no longer boots and is hard-bricked. So Odin mode is dangerous. Because of this dangerous nature of Odin mode, Odin mode is usually used to only reflash main system or recovery (see warnings below) and if possible, recovery mode is used for flashing (as it is less dangerous).

Source: XDA