Hello,
Contain means to resize the background image to make sure the image is fully visible https://www.w3schools.com/cssref/css3_pr_background-size.asp
How to set background options to contain you can check here http://prntscr.com/jaq8cy
More information about Revolution slider responsive settings you can find here https://www.themepunch.com/revslider-doc/slider-setup/#slidelayout
Regards
