Solid state physics is the study of solid materials. It makes use of various methods in physics, including quantum mechanics and crystallography.
Solid state physics does not have as much visibility to the general public compared to areas such as particle physics or astrophysics - you may not have heard of it before.
However it is the most widely practiced area of physics, and it underlies most—if not all—of modern day technology, including mobile phones, LEDs, solar cells, electric vehicles and quantum computers.
It is closely related to the scientific discipline of Materials Science, and it sits within the area of physics called Condensed Matter (which includes both solid and liquid phases of matter).
To learn more, I’d highly recommend reading the following articles:
To understand some of the key topics that are exciting scientists working in this field, take a look at the various “condensed matter” news items listed in Phys.org: https://phys.org/tags/condensed+matter+physics/
Other interesting articles: