What lossless trimming means
Most online trimmers re-encode your video on the way out, which throws away quality even if the file looks fine. This trimmer uses ffmpeg's stream-copy mode: it locates the nearest keyframes to your start and end points and packages only the bytes between them. The video and audio tracks are bit-identical to the source between those points, so the trim is as fast as a file copy and as faithful as the original.
How to choose precise trim points
Stream-copy can only cut on keyframes (typically every 1–2 seconds in modern recordings). If your start point lands between keyframes, ffmpeg rounds backwards to the previous one — so the trimmed clip may begin a fraction of a second earlier than the timestamp you typed. For most social-media use this is invisible; for frame-perfect cuts, use a desktop editor.
When to use this tool
Trim a long screen recording down to the part you want to share, cut a few seconds off the start and end of a phone clip, or pull a single highlight out of a longer video. The result is the same container as the source, so it stays compatible with the editor or platform you were going to send it to.