Hello Marcelo11756,
I am assuming you are referring to the checkbox "Delete existing occurrences on insert" which you can find when you Add/Edit a Work Order in the Field Services Add-On.
Basically it means it will delete any other occurrences which you may have scheduled for this work order and replace it with the new dates you are inserting.
For example, let's say you put in a recurring work order to go from Mar 1-Mar 20th, 2-4pm every day.
If you go back into that same work order and insert dates and times with that box checked, the other occurrences will be deleted and the new occurrences will be put in. So ALL your occurrences for Mar 1-Mar 20th, 2-4pm will be deleted if you decide to insert Recurring Dates for April 1-5th instead.
Hope that makes sense!