Also called: trigger, tag trigger, firing trigger
Triggers come in two halves: the trigger type (what to listen for) and the filters (which occurrences count). A Click trigger with the filter "Click Classes contains add-to-cart" fires only on the buttons you meant, not on every click on the page.
Exception triggers do the opposite: attached to a tag, they block it even when a firing trigger matched. This is how you keep a conversion tag off internal traffic or staging hostnames.
Custom Event triggers — matching on an event name your site pushes into the dataLayer — are the most reliable kind, because they depend on your own code rather than on markup that designers can change.
Over-broad triggers are the most common cause of inflated conversions: a purchase tag on an "All Pages" trigger fires again on every refresh of the thank-you page, and no report will tell you it happened.
In descending order of likelihood: the trigger conditions do not match (check Preview mode for the actual values), the tag has an exception trigger blocking it, consent settings are suppressing it, or the dataLayer event fires before GTM has loaded.
A tag is the code that runs; a trigger is the rule that decides whether to run it. One trigger can fire many tags, and one tag can be attached to many triggers.
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