Videos must be placed on an online sharing service and linked into your post. They can't be uploaded directly into a post as can a photo. Majority of people here use YouTube for videos. The site software recognizes standard YouTube video URLs and automatically embeds them into the post. You'll see (and can play) the video in the post preview if it embeds correctly. Best to do the post on a computer, YouTube may recognize mobile devices as such and code the video URL accordingly, which doesn't always embed properly.