Command Query Responsibility Segregation is a simple concept. When you want something to happen, you will use a Command. If you need results returned, you will use a query.
An over simplified example of this would be if you need to write to a database, you would use a Command. If you need results from a database, you would use a Query.
Installation
Symfony Bridge
Installing the Symfony Bridge will add additional functionality to the CQRS component. This can be done quickly and easily. The documentation will point out where this is required.