I'm assuming you're referring to the chart in the main admin control panel?
The two main analytics tracked are pageviews and unique visitors.
Page views are fairly self-explanatory, one page load or transition to a new page counts as a new page view.
Unique visitors is based on the IP address, and so every time a new IP address shows up in that hour, the count is incremented by one. We also track unique visitors on a daily, weekly, monthly, and all-time basis, for the numbers here:
The "registered", "guest", and "bot" variants for pageviews increment alongside each pageview, but depending on whether the user is logged in (or not), or whether that request came from a bot.
"registered" and "guest" should add up to the total pageviews, since you're either registered or not.
"bots" are usually also guests, but we use some basic logic to detect whether they're a bot or not (probably user-agent string)
For a variety of reasons, the "bot" count is not always predictable, since some bots are rather obscure, or fudge their user-agent strings, so take this number with a grain of salt.
I hope that answers your questions, let me know if there's anything else you'd like to know.