Here i am explaining you what actually YouTube Processing means.
You have Seen YouTube player giving us Video at different Resolutions like 720p,1080p,4K and Lower Qualities like 144p.Even if You Upload 4K video,You can See there 144p video Quality Options. This all is possible due to YouTube processing.
Why YouTube Processing ?
It is not possible to make your Original Uploaded Video played on Every Devices.
You uploaded 4K video having 1GB Size. Is it possible to play 4K Video on Small devices which takes huge amount of Data Usage. Definitely Not...So YouTube converts your any Video in their Sizes to have Perfect user Experience.While Processing,Your Video will be sent to youtube servers and YouTube converts your Video Quality to lower than that of Your Original and keeps Original Video Converted too with small Size..
YouTube processing is the time interval after YouTubeVideo uploading completes where uploaded videos are converted into YouTube Videos with Qualities like 480p,720p,1080p,4K e.t.c
YouTube Processing Explained ByMarques Brownlee