Trigger Time Frame is the time frame that the trade you have taken has been assessed with your entry and stop being calculated using this particular time frame.
For example:
You may identify a Falling Wedge on the 4hr Time Frame. However, you wait for the pattern to break, then enter on the 1hr retest of the horizontal resistance level.
In this case, the Trigger Time Frame would be the 4hr, even though your entry was on the 1hr, because your stop placement, presumably, would be based on the 4hr time frame.  The entry price may be slightly more advantageous as you have taken the entry off the 1hr, however, as your Stop placement is based on the 4hr and the pattern is identified on the 4hr, that is the dominant time frame for management of the trade.