bajo, adjective, adverb:
soft, faint

In an earlier Word of the Day we saw that bajo refers to something which is low in height:

una silla muy baja
a very low chair

You also use it to describe sounds which are soft or faint:

su tono bajo y pausado
his soft, measured tone

Bajo can be used as an adverb with verbs to do with movement:

El avión volaba muy bajo.
The aircraft was flying very low.

and verbs to do with sounds:

Hablaba muy bajo.
He was talking very quietly or softly.