I have create BT filter by Web Request - Parameter Value, and I create threshold on Purepath Response Time as shown below
But I confused with that threshold, why not match with fact. I try create a traffic light, but when the value of average response time 336 ms, the traffic light show severe icon as shown below. It should just show the warning icon, same as the threshold.
Whats wrong with my threshold ?
That's very likely because your severe threshold has been violated somewhere within the dashboard's timeframe (30 minutes according to your screenshot). You have to understand that by default all chart types (inlcuding traffic light) will be looking at the timeframe selected either in the dashboard or the dashlet (if it overrides the dashboard one).
As Enrico is saying, a traffic light chart will only indicate the threshold breach status for the entire timeframe that your looking at. So if your chart has a timeframe of 1 hour, if your measure hit the severe threshold at any point during that hour, your chart will show red X, regardless of what the current value for your measure is. You can view this much easier if you put in parallel a line chart, showing the threshold area so you can compare.
The only other option is to create an incident based on this measure. The incident chart shows 2 perspectives: the big area at the top represents the current value of the threshold breach, but you also get an area underneath it representing the value for the entire timeframe (like what the traffic light chart is doing):
Maximum and Average just refer to how you want to calculate the value of your measure when there's more than one value in a data point. For example, if each data point (resolution) on my chart is 10 minutes, and I get 20 values for my measure in those 10 minutes, what do I want that data point to represent? (average of the 20 values, or max of the 20 values, etc.). Then the threshold of the measure is applied against the value of that aggregation.
A traffic light chart doesn't indicate whether the average or the maximum of the entire timeframe is below or above threshold. It simply indicates, if at any point in time inside that timeframe was the threshold ever breached.