Very roughly speaking, it's a timeline where the following processes took place:
-> Alexander the Great's son Alexander survived the assassination, and fled with his mother to Cyprus - thus his line continued even after the fall of Macedonia to the Romans
-> The Hellenic empires of the East were far more durable; Antiochos III conquered India, and his successors managed to even conquer China and invade Japan
-> The Roman Empire took a very different way, dynastic-wise; Octavian Augustus married a female scion of Alexander the Great, producing a legitimate offspring, who ascended the throne in Tiberius' place
-> Constatnine the Great has never been a full emperor; instead, he was co-emperor of the East. He still finds Constantinople, but doesn't make it capital. Yet.
-> Christianity never took root in the Empire; Constantine did convert, but with his line never taking power (instead they recieved Britannia), Sol Invictus is still strong
-> Empires divided on religious base; Rome accepted christianity, while Constantinople remained true to Sol Invictus
-> Christianity never became a significant force in Europe; instead, it fractured and divided
-> Islam had a boomstart like in our history, but it got defeated by Rome; fragmentation and splintering follows
-> Persia has been restored by Sassinids as in our history, but no strong islam = no threat to its existance
-> Britannia got subdued by Lucius Artorius. He is OTL king Arthur, but his deeds are devided between his scions (consult the blood trait for more info)
-> The Vikings have been far more influential than in OTL
-> Charlemagne's empire had a strong counterbalance in Roman Empire, so it never achieved its OTL apex; later on it dissolved into a collection of petty states. Also, the Merovingans are still kickng around
-> No christianity as unifying factor = no German unity; instead, every Germanic tribe is separate
-> Don't ask me why the world is so fractured in 1066; Shay's timeline ends at the beginning of 11th century, we have at least 30 non-described years. Same goes for the Kingdom of Jerusalem
Hope this helps