The basic idea of NaNoWriMo is that you write about 1667 words every single day for the entire month of November. If you do exactly that, you'll complete a short novel of 50,010 words by the end of the month.
A novella is a very short novel or an extra long short story. Simple as that. They are normally anywhere from 20,000 to 45,000 words long and many consider a novella a quick read.