In short, the higher the difficulty level you choose, the more detailed the tab you'll get.
How to choose a tab difficulty?
If you want to play "Stairway to heaven" but you're not a Jimmy Page yet choose the Beginner.
Feeling more confident? Go on to Intermediate.
If you think you're that good choose Advanced.
Submit your own tab
When you're about to submit your own tab don't forget that you should state the Tab's difficulty and not the difficulty of the song itself. Make sure to estimate your tab's difficulty objectively. No offense, but during the first few months you're unlikely to come up with something extremely hard.
Absolute Beginner/Beginner - perfect for beginners. Simple riff, unpretentious solo and etc. Tabs that are indeed not complicated.
Intermediate - We're mixing things up a bit here. Detailed tune's parts - intro, solo, chorus, outro. Don't mess with these tabs if you have no idea what the following words mean. - hammer-ons, pull-offs, arpeggio, palm muting, pinch harmonics and etc. The perfect example of this difficulty level is "Stairway to Heaven".
Advanced - It will be quite hard. These are complicated and highly detailed pieces. Tapping, tapping with both hands, speed soloing and more hardcore madness. Will your delicate fingers be ready to take on the hellish cords?
Note: Remember that it's not strictly necessary to state the tab's difficult when you're submitting a tab. If you're unsure it's better to submit a tab with no difficulty level.