Obviously, the site awards you "Xper" ("expert" or "experience") points for various things: Asking questions, answering questions, being selected as Best Answer, and even points for things like filling out your profile.
The rankings simply show how many people have more points than you in various categories.
Your overall rank is 353, which means there are 352 people who have more Xper points than you have.
Your overalls rank is 163, which means there are 162 girls with more Xper points than you.
If you go into topic categories such as "Dating", you'll have a ranking in that specific category. If you ask or answer a ton of Dating questions, you might have a high rank in Dating, but a low rank in Health (because you rarely answer question in Health), for example.
Obviously, the Xper points and the rankings don't really mean too much other than you have been active; activity doesn't necessarily equal quality or value, and some people answer a ton of questions with nonsense just to increase their Xper points and rank. It's fun, but don't put too much importance on it.