if an emotion or a feeling deepens, or if something deepens it, it becomes stronger

Warships were sent in as the crisis deepened.

to become deeper; to make something deeper

There were plans to deepen a stretch of the river.

to improve your knowledge or understanding of something

an opportunity for students to deepen their understanding of different cultures

if colour or light deepens or if something deepens it, it becomes darker

if a sound or voice deepens or if you deepen it, it becomes lower or you make it lower

if your breathing deepens, you breathe more deeply than usual

LEARNit Dictionary