Fit
Use the Fit operation if you want to resize your image to a specific width x height (in pixels) while keeping proportions and fill the background with a specific color. (Default color is black and default gravity is Center)
Examples
Original image

1280x832
326 ko
Fit 400x600

400x600
101 ko
Fit 700x200 + Background

700x200
85,7 ko
Fit 700x200 + Background + Gravity

700x200
85,4 ko