Trend analysis is a technique to predict the future performance of a project (when it comes to both the cost and the schedule) based on its historical performance.
For example, if the project is adding an extra 2% every month to the project costs (above the budgeted costs), then we will have a clear idea how much the project will cost when it's done.
Also, if the project is behind schedule for a week each and every month, then we will be able to guess that the project will be late for a full month if it's a 4 months project (1 week delay for every month).
Of course, trend analysis is done in a scientific way (which is much more complicated than the examples above), but the way is done is outside the scope of my answer...
Trend analysis is very helpful in reporting about the project status and predicting its performance, as well as managing the expectations of the stakeholders and the client.