Events on Time Locked Tracks are fixed in time. This means that even if you change the tempo, the Events will stay at the same position, time-wise. For example, an Event that played back ten seconds in from the beginning of the Song will still play back ten seconds in from the beginning, even if the tempo is doubled.
Time locked Tracks can be used for example when you work with audio locked to film or video and mix sound effects with music in one Arrange window. Time Locked Tracks can also be used to create music with multiple tempi.