Unit studies are a special kind of curricula because they combine multiple subjects into one curriculum. You then teach all those topics by using the single unit study.
Unit studies are typically built around one topic, such as a period in history, a specific event, or even fictional or historical characters. The lessons that are built on those topics are designed to include multiple topics. For example, when my children were younger they were hugely interested in The Little House series by Laura Ingalls Wilder. One year, we used a unit study that was based on these books. Included in this curriculum were several topics including reading, grammar, history, and so on. Each lesson related to specific passages from one of the books.
There are a couple of benefits to unit studies.
One benefit is that they can hold some children's interest longer than dealing with subjects individually. For example, a child might not be interested in learning some aspect of math on its own, but placed in the context of something they are interested in, that same math might become much more interesting.
The other primary benefit is that you have a single source for several topics. This makes lesson planning and teaching easier and simpler because you have to deal with fewer resources.
Unit studies have a couple of drawbacks as well. One is that there aren't that many because it is difficult to package a variety of subjects into a single unit effectively. Another is that it can be difficult to find a single topic that will hold a child's attention throughout an entire year or even a substantial part of a year. For example, no matter how interested a child is in a particular topic, such as The Little House books, eventually, they will likely get tired of dealing with that topic, in which case the whole unit study becomes less effective.