A rollup field is a summary of related fields. Choose a relationship on the same collection, choose which field on the related collection you need to summarize and then choose how to summarize it.
An example would be if you wanted to calculate the total cost of all projects that each client/company has in the above example. You would add a rollup field to the Company collection with the projects relationship we defined earlier. The field to summarize would be Cost and you would want to SUM the costs, so you get the total sum of costs of all projects associated with a given company.