That area is called the Work Area Bar. It has a number of functions.
The most common use of the Work Area Bar is to indicate a segment of your movie you'd like to output. On many Export & Share options, you'll see a checkbox for Share Work Area Bar Only.
If i "forgot" a clip out of work area bar'
it will be not appear filme, after Export&Share ?
It will not appear if you have Share Work Area Bar Only selected.