When you mark a day's food log as "complete," you are informing MyNetDiary that you logged all food you ate during this day, regardless of how much or how little food you had. Even if you were fasting the entire day and had no food at all, you can also mark the day as "complete."
If you had a regular day and logged all of the day's meals, MyNetDiary will consider it as complete if you logged a reasonable number of calories. In this case, you don't need to mark the day as "complete," although it's still recommended.
If some of the day's meals are missing or you logged a small number of calories, then MyNetDiary would not be able to consider this day as complete and will mark it "review needed." Days with "review needed" mark are not included in the analysis and weight forecast to avoid potential distortion of analysis and statistics with incomplete data. You need to either log remaining meals and food (so that MyNetDiary will consider the day complete) or, if you indeed logged all food of the day, to mark the day as "complete."
Please note that marking a day as complete does not prevent you from logging more food - if you remembered about more food consumed, you could log it as usual.
One way to mark the currently displayed day as complete is tapping the "Complete Day" icon, which looks like a pencil, on the Dashboard. Light blue pencil color indicates that you didn't mark the day as complete, but it has enough food logged that MyNetDiary considers it complete. Darker blue color indicates days that you have marked as complete. The dialog displayed on a tap will show day completion status and will let you mark the day as complete.
Another way to mark a day as complete is using the Complete Day Food Log switch on the All Meals screen.