a timeline of faction is basically the events had placed in faction, in normal words. What have you done today, such as SAPD arrested someone or mafia robbed a bank. These are events which will be written to faction timeline everyday. timeline help anyone who read it to understand that what have been done in past days into faction, so that new players can follow up with the story. It could be the big plan of gang or history of relationship between factions.
here is a link to understand how it works, https://forum.iorp.in/topic/51/presidential-departments-timeline/3
let us know if you still have doubts.