What people are giving solutions to but haven't explained (if you care to know) is that the uncompressed video you created was so big that your computer can't read all the frames fast enough off your hard drive to play it smoothly. The video itself is likely smooth, but your computer (along with most everyone elses) can't read that much data that fast so you need to compress the video into a smaller file.
If you compress it using handbrake (or any other tool for that matter, however handbrake checks both boxes for ease of use and functionality) you'll have no problems playing it back and then uploading to Youtube.