The split timeline is basically three separate (used to be two until Hyrule Historia released) timelines because of the events in Ocarina of Time.
You have three timelines: Child, Adult, and Defeat.
WARNING SPOILERS AHEAD!!
Child Timeline: This is borne out of the knowledge Link retains when traveling back from the past. After defeating Ganondorf in his final encounter, Link gives Zelda the Ocarina of Time and she sends him back to the time that he didn't get to live. Using his knowledge, Child Link warns Child Zelda about everything Ganondorf would do, from there Ganondorf is sent to Arbiter's Ground and the events of Majora's Mask, Twilight Princess and Four Swords Adventures happen.
Adult Timeline: This is borne out of the happening of the Child Timeline. After Ganondorf's defeat, Link gives Zelda the Ocarina of Time and she sends him back to the time that he didn't get to live. However, the timeline that Link left still runs on. Later on, Ganondorf escapes the void he was trapped in and wreaks havoc on Hyrule. Since there exists no hero to stop him, the 'golden gods' flood Hyrule and the events of The Wind Waker, Phantom Hourglass and Spirit Tracks happen.
Defeat Timeilne: This truly is just a hypothetical, a what-if. If Link were to die in the final encounter with Ganondorf, the sages would still trap Ganondorf and the events of ALttP, OoX, LA, LoZ and AoL would happen.