MKV files won't play on an iPhone, iPad, or many smart TVs because Apple and most TV apps simply don't support the Matroska (.mkv) container. Converting the MKV to MP4 makes it play everywhere, and you can do it in your browser without uploading the file.
Why MKV won't play on iPhone or TV
MKV (Matroska) is a flexible open container popular for downloaded video, but Apple's ecosystem and many smart-TV and streaming apps never added support for it. The video inside is usually perfectly fine — it's the container that's being rejected.
The fix: convert MKV to MP4
Convert MKV to MP4 and it plays on iPhone, iPad, Apple TV, and virtually every device. It runs on your device in the browser — nothing is uploaded.
Will converting reduce quality?
Converting to MP4 is visually near-identical at a good setting. If the file is also large, you can compress the video at the same time to fit a device or an upload limit.
Subtitles and multiple audio tracks
MKV often bundles subtitles and several audio tracks. A straightforward MP4 conversion keeps the main video and audio; if you rely on embedded subtitles or an alternate language track, keep the MKV original as well. For the broader guide, see why won't my video or audio file play.