What are the arc regions?

A circular collider is in general not a perfect circle, but made of arc regions - meant at bending the beam with magnetic dipoles - and straigth sections, situated on both sides of the interaction points. This configuration is needed to protect the detectors placed around the interaction points from the synchrotron radiation [*] that would arise from bending the beams.

[*] Synchrotron radiation is the emission of photons from a charged particle moving in a magnetic field.