In general terms, an iCalendar (known to all as iCal) is a file format for computers, usually with the extension .ics (but we can also find it with different extensions, such as .ical / .ifb / .icalendar) capable of representing information aggregated on a time basis, in our case in a calendar.
In some cases the iCal can be used as a worthy replacement to synchronize your PMS with an online distribution portal, agency or tour operator.