Timelines are a way to cope with groups splitting up, or wanting to have multiple events occur at the same time.
They basically isolate certain chat into a given timeline, so for example if you were to have a group of 4 players and 2 went to storm a castle and 2 went to round up the towns folk, rather than muddling both conversations together in one place you can split the rounding up the townsfolk into its own timeline for just the 2 players doing that, then close it and move back to the main timeline with them once the group meets back up.
It is also a handy way of dealing with additional content to supplement the main chat, so for example the group all goes to sleep at camp but 2 players want to have a discussion. Rather than keep everyone waiting on that discussion it could be taken to a timeline where they can continue the discussion, but the main timeline can continue while this occurs. This way you can keep the story progressing when needed but not lose out on those in depth RP moments.
Finally you can close timelines to stop input after the story has continued. You can also delete timelines, however it will merge all content from the timeline into the main timeline so no content is lost. This being said you do not even need to use timelines if you do not want to, they are just another way to simplify certain scenarios.