What does the numbers mean?
...
They're codes that classify doujin (Japanese porn comics/ manga).
The idea is that they are put after a url, such as:
'nhentai.net/g/
222871'
Which will navigate you to the first doujin on the list. I don't know if the system is universal for all doujin sites - it's likely only something done on western sites as well - but, nonetheless, it's a simpler way of classifying them than trying to remember the Japanese names or their translations.