When I'm playing movie on my lg smart 49 inch TV it doesn't shows me the full screen .why?

What are you playing the movie with?
If the source is not connected with HDMI you would need to set the aspect ratio to 16:9.
If the movie is not 16:9 then you would see bars either on the sides or on the top and bottom of the screen. Most TVs have a button on the remote to either stretch the picture or cut off the sides to get the movie to fit the screen. Often looks bad and you lose some content.