This is the official documentation for remote event receivers, in which they have mentioned about after and before events too. Kindly have a look at all the sections.
Before events are triggered before the SharePoint infrastructure does
any of its own handling of the event (including committing changes to
the content database). In SharePoint custom before event handlers
always execute synchronously. Among other purposes, they can be used
to cancel the event. For example, if an add-in has a function for
deleting a list, a handler for the list deleting event can cancel the
deletion if certain conditions are not met. If the event is part of a
sequence of events, cancelling it prevents any of the later events
from occurring at all. For example, if your handler for the ItemAdding
event cancels the event, the ItemAdded event, which normally comes
later, is not triggered.
After events are triggered after the SharePoint infrastructure does
any of its own handling of the event. In SharePoint, remote after
event handlers, for list and list item events, always execute
asynchronously. (App events are an exception.) Among other purposes,
they can be used to log events.
Your add-in can handle the following list and document library events. Events ending in "ing" are before (synchronous) events and those ending in "ed" are after (asynchronous) events.
Before (synchronous) After (asynchronous)
ListAdding ListAdded
ListDeleting ListDeleted
FieldAdding FieldAdded
FieldDeleting FieldDeleted
FieldUpdating FieldUpdated
The field update events are about changing the properties of a field (column) on a list, such as whether it is sortable, not about changing the data in the field.
Your add-in can handle the following list item events.
TABLE 2
Before (synchronous) After (asynchronous)
ItemAdding ItemAdded
ItemUpdating ItemUpdated
ItemDeleting ItemDeleted
ItemCheckingOut ItemCheckedOut
ItemCheckingIn ItemCheckedIn
ItemUncheckingOut ItemUncheckedOut
ItemAttachmentAdding ItemAttachmentAdded
ItemAttachmentDeleting ItemAttachmentDeleted
ItemFileMoving ItemFileMoved
ItemVersionDeleting* ItemVersionDeleted*
ItemFileConverted