Abeir-Toril was the name of one of the primordial planets that formed upon the creation of Realmspace. The two planets were eventually split in twain, forming the separate planets of Abeir and Toril. Upon their separation, Abeir was sent to a parallel dimension, and the two planets experienced separate timelines and histories for tens of thousands of years.
During The Time of Troubles the planar barriers between Abeir and Toril were weakened, allowing for limited travel between the two realms. The briefly broke those barriers completely, causing portions of Abeir and Toril to swap places and opening up large portals between the worlds, allowing travel between them.