Vacuuming

There's a script running on my server hosting a PostgreSQL database that does some type of vacuuming routine every
Fridayat 5:00 p.m.  Specifically, the script executes this command: 

     psql -d database_name -c "vacuum full verbose"

and e-mails the verbose output to me.  Today's report today contains about 900,000 characters.

I don't event know where to begin reading that report, interpreting what it's telling me, determining what's important,
orascertaining what's routing and unimportant.   

Can anybody give me any guidance or point me to a document that I should read to understand what the PostgreSQL vacuum
doesand why this is an important function and how to decipher the verbose output?