Video Output Options
The video output options mostly control how the .ass subtitles are rendered into video.

Background Color
The “Apply BG” button becomes active when any tracks are selected in the left pane. It replaces the background color or media with the selected color. A color can be typed in via RGB hex code, or can be selected with a color picker by clicking “Choose…”. The selected color also doubles as a default for any tracks that do not have a color specified when the conversion is run.
Output Resolution
This sets the resolution of the rendered video. It’s editable, so you do not need to use one of the available options.
It’s also used in the subtitle creation process to set the PlayResX, PlayResY options for the subtitle. See Aspect Ratio for more details on that.
Override background
This feature isn’t implemented yet, but when it is, it will allow for scaling and letterboxing the background video/image to the specified output resolution. Until it is available, please make sure your background media is already at the desired resolution.
Output File Type
This is the container type to use for your output file. mp4 is recommended for compatibility. If you need to ship with flac audio, use mkv.
Video Codec
The available options depend on the container format chosen. This is nowhere near an exhaustive list of the available codecs ffmpeg can do, but it includes all the best modern ones. For compatibility, h264 is recommended. If you need lossless video, e.g. to reencode or edit further before release, libvpx-vp9 is recommended.
Lossless video
Encode losslessly - only recommended if you are going to reencode or edit further. If you have enough bandwidth for it, YouTube will accept lossless uploads though.
Video Quality
If lossless is not selected, you can set the CRF quality value for the video. The default is usually a good choice. Left of that is lower quality and right is higher. The slider goes from 35-15 (CRF scale is reversed). The default is 23.
Audio Codec
Available options depend on the container format. aac is recommended here unless you want lossless audio, in which case use flac.
Audio Bitrate
Has no effect if using flac. The default is 256k, which should be more than enough for aac. Feel free to adjust if you’d like.