Using the Transcribe feature you can have subtitles automatically generated based on the audio of a video.
To have a video transcribed, navigate to the Subtitle page of the media you want transcribed.
Select the language that is spoken in the video using the drop down menu under Transcription. Press "Transcribe" and wait for the subtitles to be generated.
Next to the subtitle language there's a processing icon displayed until the transcription process has completed.
After the subtitles are generated you can open up the subtitle editor directly on the subtitles page if you want to make any changes to the automatically generated subtitles.