Think of it like a gentle fade-out effect, where the music gets softer and softer. It's the opposite of a crescendo, which is a gradual increase in volume.
Here's how it's usually written in music:
* Symbol: < (a less-than sign)
* Abbreviation: dim. or decresc.
So, if you see a diminuendo symbol in a piece of music, it means the performer should gradually decrease the volume of their playing or singing.