xbooru.com/ The numbers seen there represent the number of images currently on the website.
"Total number of visitors so far: 10,601,849" is fairly obvious.
"Serving 201,702 posts - Running Gelbooru Beta 0.2.0 " The "serving X posts part" is again, the number of images on the website. One image counts as one post. It's a unified system that runs on all boorus, it's a way to count and refer to images on the booru.
The 0.2.0 is obviously the software version we run. The creator of said version is the owner of Gelbooru, that's why it says "Gelbooru Beta 0.2.0".