Aggregates are Entities that hold other Entities and Value Objects that help keep data consistent. From Vaughn Vernon's Implementing Domain-Driven Design:

Aggregates are carefully crafted consistency boundaries that cluster Entities and Value Objects.

Another amazing book that you should definitely buy and read is NoSQL Distilled: A Brief Guide to the Emerging World of Polyglot Persistence by Pramod J. Sadalage and Martin Fowler. This book says that:

In Domain-Driven Design, an aggregate is a collection of related objects that we wish to treat as a unit. In particular, it is a unit for data manipulation and management of consistency. Typically, we like to update aggregates with atomic operations and communicate with our data storage in terms of aggregates.