Denormalization means adding redundant information into a database to speed up reads.
Denormalization = avoid joins by repeating the information.
Joins in a relational database such as SQL can get very slow as the system grows bigger. For this reason, we would generally avoid SQL joins. Denormalization is one part of this.
For example, imagine a database describing projects and tasks (where a project can have multiple tasks). You might need to get the project name and the task information. Rather than doing a join across these tables, you can store the project name within the task table in addition to the project table. Or, you can go with a NoSQL database. A NoSQL database does not support joins and might structure data in a different way. It is designed to scale better.