Search the forum, this has beena answered extensively before.
pconnect opens a new connection or re-uses an existing connection, and doen't close the connection when your script exits (not even if you use the close function!)
Advantage: no delay for opening a connection (when a connection is available)
Disadvantage: eats memory, because connections taht are not used for X minutes are kept open.
Like I said, search the forum, there are better explenations than this one.