Fast rotation without quality loss
MP4 and MOV files store which way up the picture should be shown. For a plain rotation the tool only changes this setting and copies the video as it is, so even a long recording is done in seconds and keeps every detail. Phones, browsers, Windows, macOS, VLC and messaging apps all respect it.
A few older programs and some editing tools ignore that setting and still show the video sideways. For them, choose Re-encode: the picture itself is turned and saved again, so it looks right everywhere. This takes longer, depending on the length of the video and your device.
Mirror and upside down
Mirror (left to right) fixes front camera videos where text reads backwards, and Flip upside down turns a recording that was made with the phone held the wrong way. Mirroring changes the picture itself, so the video is always re-encoded; the sound is copied unchanged when possible.
WebM and MKV files cannot store an orientation, so they are re-encoded for every turn. MP4 and MOV results use H.264 video and AAC sound, which play on practically every device.
See it before you save
The preview plays the video already turned and mirrored, and shows the size of the result, for example 1080×1920 for a portrait phone video. Use the turn buttons to step 90 degrees at a time until it looks right.