Time data is the main subject for performance analysis. The majority of filters in Timeline Viewer make sense only when the Time analysis subject is selected. For example, the Thread States filter shows you the states' length (in ms) for selected threads; Call Tree shows how long certain calls run, and so on.
The value shown next to the Time analysis subject is the total length of time intervals selected in Timeline Diagram. For example, your application has three threads: Thread#1, Thread#2, and Thread#3. Profiling lasted 50 ms. You want to analyze only Thread#1 and Thread#3. After you select them in Timeline Diagram, the Analysis Subject filter will look like follows:
The resulting selected time interval is the sum of the Thread#1 and Thread#3 length: 50 ms + 50 ms = 100 ms.