Hi Johnny and welcome aboard!
Get there in good time - There will be a pre-run brief that explains procedures, or feel free to have a quick chat with one of the volunteers, who will be wearing high-viz vests.
You won't need your barcode till the end, so put it somewhere safe ( like a zipped pocket).
Everyone starts at the same time, and the timekeepers will press "GO" on their stopwatch. As you cross the finish line, the timekeepers will press a button that records the time, and you will be given a position token. At that point you need to retrieve your barcode, take it and the position token to the scanner, who zaps both these. Then you go and have a cup of tea and a natter with the others.
Later that day, the magic parkrun elves will have matched up the info from the stopwatch, the barcodes and the position tokens, and will send you an email (or text if you've registered for that) telling you your official time. You can then spend hours looking at the results, event history, age graded percentages, and work out how long it'll be till you've earned a milestone T shirt 