Mousekeeping = Housekeeping
Mousekeeping envelopes = Envelopes decorated with a Disney theme which has cash as a tip for the person who tidy's up and cleans the WDW resort room.
Some people use a computer to print out the Disney theme pattern on the envelope- you could also use Disney stickers or draw a character yourself if you are talented.
Tips tend to be $1 a person per day for the envelopes.
We are a family of 4 and tip $5 per day.
I hope the above helps.