Simply put, a post-hypnotic trigger is a response to some sort of stimuli after a hypnosis session that causes a subject to react in a predetermined, suggested, way.
For instance, if after induction, you implanted a deep hypnotic suggestion that every time they hear you, and only you, say the phrase “orange pickle” they would immediately forget their name. If embedded well enough, usually with repetition and reassurance that their name will come back to them and it’s just gone temporarily, you can wake them from trance and test your phrase out.
The suggestion is what they say, think, or do based on your trigger, which is the action point on which they react.
Post-hypnotic triggers can be nearly anything you can imagine, a sight, sound, a touch, a series of words, a picture. It’s really up to the imagination but to ensure a lasting post-hypnotic trigger one must consider the environment the person will tend to be in. For instance, a truck driver with a smoking problem may see a lot of stop lights or stop signs on his/her daily route. Implanting the suggestion that every time they see a stop sign/light they would notice how fresh the air is, how alert and in control they feel, and that they have the strength to make it through the day without smoking. If you said every time they enter a door, this would not be as effective nor lasting.