The server is delivering only part of the resource, in response to a Range request from the client.
206 Partial Content is what makes resumable downloads and video seeking work. When a client sends Range: bytes=1024-2047, the server responds with 206 and just those bytes, plus a Content-Range header describing what was sent and the full size. Browsers use 206 for video scrubbing; download managers use it to resume interrupted transfers; cloud storage clients use it to parallelize large file downloads.
When servers should return it: Return 206 only in response to a Range request, and only when you can satisfy the range. If the range is invalid, return 416 Range Not Satisfiable.
curl -i -H "Range: bytes=0-1023" https://example.com/video.mp4
HTTP/2 206
content-range: bytes 0-1023/5242880
content-length: 1024
[binary bytes]
For a one-page reference of all HTTP status codes, see the HTTP cheat sheet. For testing API responses, try the API Tester tool. For inspecting responses on the command line, the curl cheat sheet covers the most common flags.