What's Peers in Utorrent?

Torrent peers are the people currently downloading the same torrent you're downloading. When they download a portion of said torrent, they help you download what they have already downloaded.

How do seeds and peers affect download speed?

In general, the number of seeds and peers are directly correlated to how fast you can download a file. The more seeds mean that you can download files much faster since you have more sources to concurrently download from. With peers, it's the opposite.

What is a good seed peer ratio?

There really isn't any "optimal" ratio of seeds/peers. You could max out your download bandwidth from one single seed, regardless of how many peers or leechers there are. In general, the more seeds, the better, as this gives you more places to connect to to download the file.