Well, I'm inclined to agree with Drainy, that you have a typo and you're referencing Timeline instead of FlashTimeline.
However... Timeline is a Sideview Utils module, so the other possibility is that you've copied a view from some Splunk instance where Sideview Utils is installed, onto a system that doesn't have Sideview Utils. If so, installing Sideview Utils will make the error go away.
Although Timeline was introduced way back in 1.2.5, it wasn't used in any non-prototype views for quite a while.
As to what it is, it provides the same kind of functionality as FlashTimeline except as a compact heatmap bar instead of a full histogram. You can click, drag, shift-click bars and it will select the corresponding timerange. It consists of a few rows of bars where the top row is the most granular, for example minutes, and the row below that is hours. You can also interact with the lower rows to select timeranges.
Anyway, even in Sideview Utils 2.0 it's still marked as a prototype. Meaning that I will most likely change the HTML and CSS structure significantly before I document it and release it officially.
Note that besides the names being similar and the view referencing the module, the flashtimeline.xml view and the FlashTimeline module are not the same thing. If copying over flashtimeline fixed it, then it sounds like someone had modified the flashtimeline view. Perhaps they were indeed trying out the Sideview Timeline module as an alternative to Splunk's FlashTimeline module. Yes it is very confusing that there is both a view and a module with the same name. 😃
Actually no, I have two xml files in my view, flashtimeline and timeline, timeline.xml came with Splunk 4.3.2, I had to copy the flashtimeline from an older version of splunk to remove that error or warning message and use the flashtimeline.xml file instead