Editing & Post-Production
Export
Also called: render, render out, output, encode
The editor plays back a sequence of references to source files, effects and adjustments. Exporting flattens all of that into one file a player can open, which is why it takes time and why the settings you choose are the last chance to affect quality.
For YouTube, an MP4 container with H.264 video and AAC audio is the safe route, at the same resolution and frame rate you shot in. Give it a generous bitrate, because YouTube re-encodes everything on upload and starts from what you hand it, so a thin export gets thinner. Blaming YouTube's compression for a soft picture usually means the export was the problem.