a little doubt

Does someone could tell me what means this box, in the picture?

Someone could tell me?

It appears to be a ‘Render Border’. You probably hit Ctrl-B to do a select instead of just ‘b’. To clear this, use Ctrl-Alt-b or unclick ‘Border’ in the Render Properties under Dimensions.

BTW, this is a useful feature on large images. Use it in camera view to render only the area inside by the render border.