The trigger point defines when the trigger should be called. A trigger may be called at many different points.
Before and After Open/Close table:
Before and after open/close triggers are invoked immediately before or after an attempt is made to open or close a table or index.
Before and After Read (Input):
Before read triggers are invoked immediately before an attempt is made to read a record from a table, and therefore have no access to information from that record. After read triggers are invoked just before the details are passed back to the invoking function
Before Insert, Update or Delete:
These before action triggers are invoked immediately before an attempt is made to insert a new record into a table, update an existing record in a table, or delete an existing record from a table.
After Insert, Update or Delete:
These after action triggers are invoked immediately after data manipulation attempts are made (as described above). The after update trigger has access to both the old and new contents of the record.
The trigger point is a very important consideration. When before action triggers are invoked, it is not guaranteed that the action will be successful. You must always consider possible failure.
Trigger Points in the Technical Reference Guide