Purpose of IAggregateRoot interface in in Microsoft Repository Pattern document

I am looking at the Respository Pattern in Microsoft website.

Respository Pattern

I section i am not understand what IAggregateRoot does?

public interface IRepository<T> where T : IAggregateRoot
    {
        //....
    }

Can each respository class interact each others e.g. Product respository may be used by OrderRespository.