ceck the faq on bitcoin wiki
What is "difficulty"?
Difficulty is a measure of how difficult it is to find a hash below a given target.
The Bitcoin network has a global block difficulty. Valid blocks must have a hash below this target. Mining pools also have a pool-specific share difficulty setting a lower limit for shares.
https://en.bitcoin.it/wiki/Difficultyif you have more question about bitcoin, i prefered you to read this article before ask again
https://en.bitcoin.it/wiki/Help:FAQyou can use this to calculate your profit (like virosaGITS suggestion) :
https://bitcoinwisdom.com/bitcoin/calculator (Full)