Eye boogers are the excretions eliminated from the eye gland. In the daytime, there are some dusts coming into your eyes. And once you cry, the tear may be evaporated into residue. Then in the night when you sleep, the grease inside will mixed with the residue and the dust to come into the eye boogers.
Eye boogers are the dust from the eyes, which would be appear on the eyelid or in the corner of the eye normally in the morning. It comes from the tears that are made of water, oils, protein and mucin. The main function of tears is moistening eyes to protect them. When you are sleeping, the eyelids aren't blinking. Hence, the dirt and debris are thick without washing by tears, so that the eye boogers are easily accumulated on the outer corner of the eyes.
In simple words, eye boogers are some dust which comes from your eyes. When you get sleep, you close your eyes, so there's no chance for the tear to wash the dust. At this moment, the dust will become the eye boogers which can exist in the corner of your eyes when you wake up in the next morning. You should not worry too much about the eye boogers, because it's just a kind of normal symptom. They can also clean your eyes too. On the other hand, if you have too much eye boogers, you may have to go and see a doctor so that you can make sure what happened to your eyes.