Graha Bhedam in Carnatic music is the process (or result of the process) of shifting the Tonic note (śruti) to another note in the rāgam and arriving at a different rāgam.
Graha literally means position and Bhedam means change. Since the position of the śruti is changed (pitch of the drone), it is also sometimes called Swara Bhedam or Śruti Bhedam though Śruti Bhedam and Graha Bhedam have some technical differences.
Read more about Graha Bhedam: Definition, Melakarta Rāgams, Janya Rāgams