Your food unit target refers to how much of each food group you can have each day.
Each food group target is a maximum (except for vegetables which you can view as a minimum). See below the graph in the 'food and exercise diary' that will indicate your daily units. (Left side = how many units you have used for that day based off what you have entered in the diary/the right side = your limit for the day).
If you eat more in a day than suggested, just start again immediately from the very next meal. The more consistently you follow your eating plan the quicker you’ll get results and feel better.
That said, it’s normal to have a few days when you go over, so keep everything in perspective.
And remember: it's what you do most of the time, not some of the time, that counts.