Automatic transcoding is the process of converting a single uploaded video file into a ladder of versions at different resolutions, bitrates, and formats — typically 360p, 480p, 720p, 1080p, and sometimes 4K, encoded in modern formats like H.264 and AV1. The video player then picks the version that fits the viewer's connection and device in real time, a technique called adaptive bitrate streaming.
I lean on transcoding for any client whose audience watches on mobile or in places with patchy connectivity. The alternative — serving one large file and hoping the connection holds — is what produces the spinning-loader experience that pushes viewers off a page. With transcoding, a viewer on a slow connection drops to 480p instead of buffering, and most won't notice the resolution change at all.
The other reason to transcode is page performance. A single 4K source file is wasteful when most visitors will watch on a phone, and the larger the file the worse it hurts Largest Contentful Paint when the player initializes. Adaptive streaming sidesteps that by starting low and stepping up.
Most mature video hosts handle transcoding for you. VideoPress, Wistia, Vimeo, Mux, and Cloudflare Stream all generate the renditions automatically on upload. Self-hosted setups can do the same with FFmpeg in a queue, but the operational overhead — encoding compute, storage costs, the manifest file — is rarely worth it unless you have an unusual reason to keep video off a vendor.