Debajo "below" is an adverb, indicates an object position referring to other. Abajo "down" is an adverb too, we usually write "abajo" to talk about places.

Let me add something: if you are indicating the position of something, debajo y abajo can be interchangeable, and in English they can be translated as 'under'. 1. Abajo de la mesa, 2. Debajo de la mesa -> 'under the table'.