The trace window displays historical information on the instructions executed by the target.
The type and number of the trace entries depends upon the target that is connected when gathering trace information. Some targets may trace all instructions, others may trace jump instructions, and some may trace modifications to variables. You'll find the trace capabilities of your target on the shortcut menu.
Each entry in the trace window has a unique number, and the lower the number the earlier the trace. You can click on the header to show earliest to latest or the latest to earliest trace entries. If a trace entry can have source code located to it then double-clicking the trace entry will show the appropriate source display.
Some targets may provide timing information which will be displayed in the ticks column.
The trace window is updated each time the debugger stops when it is visible so single stepping is likely to be slower if you have this window displayed.