If your video file is named something like MovieName.1080p.BluRay.x264 , look for a subtitle file with a similar "tag" (e.g., BluRay or WEB-DL ) to ensure the text lines up perfectly with the audio.
Another highly-rated Android app that automates the matching process based on your video's specific file name to ensure perfect timing. 2. Best Online Search Portals
Likely what you're referring to. It lets you search for subtitles by filename or title and downloads them in .srt format, which works with almost any player.