.container has one fixed width for each screen size in bootstrap (xs,sm,md,lg); .container-fluid expands to fill the available width.
So for example, say your browser window is 1000px wide. As it's greater than the min-width of 992px, your .container element will have a width of 970px. You then slowly widen your browser window. The width of your .container won't change until you get to 1200px, at which it will jump to 1170px wide and stay that way for any larger browser widths.
Your .container-fluid element, on the other hand, will constantly resize as you make even the smallest changes to your browser width.