a trigger is a rule that you put on a table which basically says, whenever you DELETE, UPDATE or INSERT something in this table, also do something else. For instance, we might want to log a change, but instead of writing two separate queries, one for the change, and one for the log, we can instead write a trigger that says, “Whenever this row is updated, create a new row in a different table to tell me that the update was made”. It adds a little overhead to the initial query, but since there are not two packets traveling to your database to do two separate things, there is an overall performance gain (in theory anyway).
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