When placing in a video, you can choose whether or not to crop it to fit the exact shape of the interactive region. This option is available as a check box above the text-entry field for the share link to your video.
What cropping will do is trim off the sides or top & bottom of the video as needed to keep as much of the video as possible, but fit it exactly to the dimensions of the interactive region, and keep the video centered. However, if there is any relevant imagery or text that goes to the edges of the video, those may be cut off or cut out entirely when cropped.
In the case that you do not want your video cropped, you can leave the box unchecked. This will fit the entire video into the interactive region, but it will place black “letter-boxing” bars into the areas that the video can not cover, similar to a wide-screen TV effect.
Please note that currently, all video links can be cropped with the exception of links shared from Vimeo. those will always remain uncropped, and will use the settings of the video on Vimeo to display.