选择你想看的视频。 点击“音频和字幕”选项。 选择你想要使用的字幕。 按“返回”回到“描述”界面。 开始播放视频。

I'm interested in that, but I only have a few mins. Can you give me at most the first 500 characters of a detailed explanation in English about that?
Don't start playing it yet, as you'll be changing your subtitle options from the Description screen. If you have a Roku 3, you can access the subtitle options during playback by pressing Down on the remote. You'll find this on the video's Description page. The available subtitles are dictated by the video's creators. Your subtitle choices will be saved. Your new subtitle selection will display on the screen.