You're catching on.

As for what overrunning means, have you ever ridden a bicycle? When you pedal, it moves the bicycle. You can also stop pedaling and coast. When you are coasting there is a one way clutch in the rear wheel hub that is overrunning. The one way clutch in your transmission is the same, just larger. If there was no one way clutch in the hub you would have to pedal all the time, but you could resist the pedals turning to help slow the bicycle.

In your truck you could always start in manual 1 or 2. Usually, but not always, no additional damage will occur. The one way clutch is failed, but probably won't damage anything more than it already has.