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 tracker' 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).
Your daily units that you use will change each time you enter something into the tracker.
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.
To read more about food groups, and what your daily/weekly targets are, refer here.
And remember: it's what you do most of the time, not some of the time, that counts.