From the post creation flow, you can add a video to your post. To add a video to your post, click the Video icon just below the post Description.
The maximum video size varies by community, but may be up to 500mb.
You'll be prompted to select a filed stored on your device. After selecting the file, a preview of the video will be available in the post creation flow. You can click the X in the top-right of the video to remove it from the post. Then enter the post's Title. Additionally, you may be required to enter a post Category and Tag before clicking Submit.