Subtitles are available for most content on Nevflix. To enable them on the content you're currently watching, start by finding the subtitle button on your video player interface (under the settings menu on Plex Web player; swipe down on remote trackpad on Apple TV; should be more obvious on other devices).
Once you have them enabled, if there's a direct subtitle available it will show in the list that subsequently appears. If not, or if the direct sub option is low-quality, you can run a "search" which will utilize several subtitle databases to find corresponding sub. Sometimes the search errors out and sometimes it just doesn't show accurate results; hence why we're actively working to add direct subs to as much of our content as we can.
If you'd prefer to have subtitles enabled by default, you just have to set it once and it applies across all your devices. To do so, go to plex.nevflix.us for Plex Web, navigate to settings (wrench icon in the top right), go to Account on the right, scroll down to Audio & Subtitle Settings and hit Edit. Here you can set your preferred audio and subtitle settings, as as the Auto-Select Subtitle Mode which is what enables auto-subs. You can either set that to Shown with foreign audio, which will only auto-enable subs when necessary to understand audio spoken in another language, or Always enabled which will just automatically enable subs for all content as long as there is a direct subtitle available. If there is no direct sub available for whatever you're watching it will not work, so you'll still need to use the search option to find subs.
In that same section, you'll find SDH options. My recommendation would be to set that to Only show non-SDH subtitles for anyone that is not hard of hearing. SDH subs are subs specifically made for the hard of hearing, so they include descriptions of audio which is unnecessary for those of us that plan to watch content with the sound on. I'd also recommend setting Forced subtitle searches to Prefer forced subtitles if not already set as such.
If you notice any issues with our direct subtitles, such as being poor quality, out-of-sync, or incorrect, please report them through Plex as outlined above.