The trigger shot is given prior to IUI or retrieval to ensure the eggs mature and release within a certain timeframe (usually 36-42 hours after the shot). So basically it controls when ovulation will occur (if you haven't already had an LH surge-then ovulation might occur earlier).
In the case of retrieval, they will time your retrieval to a specific timeframe after the trigger shot so the eggs will be mature and release for them when they aspirate the follicles. For me at Cooper, they've been doing the retrievals about 33 hours after my shot but i think some places differ, and for me for IUI they always had me come in at 36-38 hours.
I also wanted to mention that I feel for you with all the prep work involved. Its very draining, the appts, the phone calls, trying to figure things out, learning the terms, interpreting the tests. I was totally overwhelmed the first time I did it. Hang in there, and good luck tomorrow (Thursday) with your followup! I will be thinking of you!