Solving a block and share relationship [pool mining]
When your share is better than the difficulty you have solved a block. Other than that, the best share is only for your information/amusement. Although, some pools only count shares above a certain difficulty towards the work one has provided.

Even succeeding at the difficulty, you could still not be discovering a block for one of the following reasons:


The difficulty you were looking at was the difficulty at which the pool is accepting shares as work, instead of the networks difficulty.
You were working on a stale block chain.
Another block was found at the same time and was accepted right before yours.