Open Odin on your computer and connect your phone via USB. The ID:COM box in Odin should turn blue, indicating a successful connection.

The exact of your Galaxy A5 (e.g., 2017, SM-A520F).

In simple terms, a custom ROM is a third-party replacement for your phone's stock operating system. The Samsung Galaxy A5 (2015) originally shipped with Android 5.1.1 Lollipop, the 2016 model received its last official update to Android 7.0 Nougat, and the 2017 model topped out at Android 8.0 Oreo. A custom ROM can take your phone far beyond these versions, to Android 9, 10, 11, 12, 13, and even 14.