The entries tag is a special tag that works only within the calendar tag - specifically within the days tag present within the calendar tag.
This tag enumerates all the pages published on the day being enumerated by the days tag. The pages will belong to the template specified by the masterpage parameter of the calendar tag.
Please see Core Concepts - Events Calendar for a working example of this tag in action.
This parameter limits the number of pages that this tag enumerates.
As it enumerates the fetched pages, by default this tag makes available all the variables associated with every page - this includes the editable regions too.
If you simply require to display the names and links of the pages, the values contained within the editable regions of the pages can be skipped from being fetched by setting this parameter. This will result in a performance boost.
As this tag iterates through the pages, at each iteration it sets all the variables that one normally finds set when that page is accessed in a page-view (see Variables available in Views).