When you're mining, you're actually attempting to brute force a hash to be less than a certain value. On modern hardware, this is generally billions/trillions (GH/TH) times per second. Bitcoin only cares about an actual solution which is less than the value required by the current difficulty.
"Difficulty 1" shares are hashes which would have met that requirement if the network difficulty was still 1. Approximately 1 in 2^32 (~4.2 billion) hashes will produce a result that is at least good enough for difficulty 1. From that point on, the easiest way to look at is is exponentially:
50% (1 in 2) Difficulty 1 results can also match Difficulty 2.
25% (1 in 4) Difficulty 1 results can also match Difficulty 4
12.5% for Difficulty 8
6.25% for Difficulty 16
....
1 in 23,844,670,039 chance of matching today's network difficulty.
Pool mining uses this as the basis for allocating block rewards. If you were solo mining, Bitcoin would simply ignore any results that are not the network difficulty (or better). With pool mining, the pool has you submit any results that match a certain difficulty (it used to be diff=1, these days it's normally variable based on your speed). Most pools target very frequent share submissions (10-30 per minute). This lets them get a fairly accurate estimate of your local hashing speed, and also allows them to give you a fair share of the reward by comparing how much effective work you have submitted with the rest of the pool's miners.
The "problem" you're being given by the pool never changes, just the quality of the answers you give back.
Okay, your opinions are noted.Thanks.