Can mean whatever you want it to mean, but the most popular uses are:
1. An exclamation akin to 'ouch' or 'uh oh..'
2. Filling in the blanks where other (rude) words would go.
3. A greeting! I personally say meep instead of Hello...
4. A random expression of happiness used to fill gaps in conversation.
Meep is the best word ever! Meep!
2. "Ahh.. Meeping Hell!" >.<
3. "Meep." ^.^
4. "....meep...." ^__^
Interestingly enough, the religious sect 'The Followers of the Meep' regard the use of the word Meep by any non-Follower as heresy.
The Great Meeper said, "This non-Follower has committed the grievous crime of Meeping without a licence."