I don't mine, but I can take some educated guesses. It is my understanding that miners generally work by having you solve blocks at reduced difficulty (say 1), and then submitting them to the pool. They verify those blocks as being correctly solved, and wait for one that is solved "well enough" to meet the network difficulty. (Currently 877226 according to http://blockexplorer.com/q/getdifficulty
). So if they had you solving at difficulty 1, and you were the only person in the pool, it would take about 877226 blocks (on average) before you got any Bitcoins. However, because it's a pool and everybody is working on the problem with differing hardware and speeds, the effort is distributed.
So in short, it depends on the number of other people in your pool and what difficulty they are solving at. These accepted hashes are your proof of work, so the more acceptances you have, the more Bitcoins you should get from the ultimately mined block. I don't know what 121 (27) means though because I don't use GUIMiner (or any miner). If you look at the documentation and find technical terms, I can probably reason out what they're talking about.
Hope that helps.