Duck and Cover

At the beginning of creating a map. What does the square with the yellow border stand for?

That is the camera bounds. Its the limit of the area that you actually can play in. To expand the area go to the "level" button and at the top left side of the screen you will have some buttons to set the top or button of the bounds.

Thank you for your answer.
So the square is the place where I make the map.
I'm sorry for asking so stuped questions for you, but I heve never created maps of FT. I have another Q.
When I place some tile set on another one it starts to blink with the red color. Am I doing something wrong?

Yep. That means the tile(s) is/are overlapping with either another tile or an entity. That's bad. The game can't draw two different tiles in the same space and overlapping tiles can seriously screw up the occulsion data (which is also bad). Rarely will you HAVE to have tiles overlap, so do yer best to fix it.

The only real exception is if the tile is overlapping with an invisible tile, but even that's bad.

There are cases where you'd think you'd need overlapping tiles but most of the time it can be avoided by doing simple tricks.