Since that definition probably doesn't help much, one way to gauge threshold effort is to monitor your concentration and breathing.
When you're running at threshold level, just maintaining your pace and form takes up so much of your concentration that you wouldn't try to carry on a conversation with a running partner. But if you had to speak, you could do so without having difficulty breathing.
So if you're breathless when you try to speak, you're probably running faster than threshold effort. If you can hum or sing as you run, you're going slower than threshold effort.
Of course, when in doubt, err on the slow side. In general, threshold pace usually works out to be right around 10K race pace.
As for training, I recommend a threshold workout once a week, even for 5K runners. The workout should consist of five to 30 minutes of running at threshold effort.
You can do this as one continuous segment or break it into repeats of five minutes or longer, with 30 seconds to two minutes of easy running after each repeat.
In your first week, you may want to start with a single five-minute threshold effort. Then keep adding a small amount of time each week to the total time run at threshold until you reach 30 minutes. If you train properly, your pace at threshold effort should get faster as the weeks go by.