If you think timeline here is the same as timline in Twitter or Facebook, well you're not 100% wrong.
In NLP, timeline is how we mentally code pics, voices, feelimgs, aromas, and sensations of experiences in past, present and future.
Our internal representation of time is another form is submodality.
Sometimes when we do something we'll be reminded of something we've experienced before.
For example, when we walk through a street that reminds us of the memories when we walked there with other people before.
We can understand timeline by experience it ourselves. Put in your mind how you get through today.
You wake up, take bath, eat breakfast, go out, come back home and sleep.
How do you see it? With pictures? Voices? Feelings? Can you still feel the taste of ur breakfast today?
Where's the picture? Right, left up or below you?
Now do that again for what you're gonna do tomorrow. You'll find out that those pics form a time line.
We all have different types of timeline. Most people have their timelines formed in "V" letter. Ssome have it with their past in the back and the future in front line. Etc, etc.
We can know other's timeline form by paying attention to the words they say. For example:
"The past is behind me now. My future awaits me ahead."
"My memories with him have gone so far away from me and they have all gone blurred."
Timeline is used to change the way we think of something (or some things) so that we can do them more effectively.
Well, the same as all methods in NLP, we gotta think of the ecology --the consequences-- when thinking about changing our/people's timelines. The first thing to do is to make sure that the outcome is well-formed.