The verb "to deepen" means to make something deeper or increase its depth. It can also mean to intensify, enrich, or expand a particular quality or aspect of something. For example, if you are learning about a subject and want to gain a deeper understanding, you would be deepening your knowledge by studying more in-depth information. In addition, the word "deepen" can refer to making a sound louder or increasing its intensity, such as when someone says they need to turn up the volume on their music player to deepen the bass.