Missed days are days you haven't taken photo on. For example, you've been taking photos in January. But didn't take a photo on January 4th and January 19th. This will result in 2 missed days.
Having any missed days means that the 365 is not perfectly complete. You can still go for 365 photos published, but with the gaps - this will not be the ultimate 365 project.
I'm taking photos every day, but am not able to upload them daily. What happens then?
If, for example you're going on vacation and won't be able to upload photos for a week, your missed days number will grow. But you can always upload those photos once you're back and bring the missed days number back to zero.