About the RFC lookup

Hi Gurus,

Please tell me What is LOOKUP AND WHAT IS RFC LOOK UP 1)HOW IT WORKS WHY WE HAVE TO CONFIGURE THE RFC ADAPTER AND IN WHICH DIRECTION WE HAVE TO CONFIGURE .

2)WHY WE NEED BAPI'S FOR RFC LOOK UP

PLEASE TELL ME CLEARLY IN GENERAL ENGLISH .

THANKS AND REGARDS,

RAM.