If I change the code the color will disappear, I do not know how does this happen.and when do we will use this features.
This is activated because of code coverage. If you want to remove it then follow these steps.
Refer the screenshot for more details.
It looks like you accidentally pressed the Coverage button instead of the Run or Debug button next to it.
To remove the red background color do the following: in the Coverage view click the Remove Active Session or Remove All Sessions button.
For information on what this Coverage does and what it is used for, see the EclEmma web site.