We can support videos with resolutions up to 1080p (1920px by 1080px), so provided that the resolution of the original file uploaded was lower than that, you shouldn't see any degradation of quality upon upload to Thinkific. (Here's our recommended specs for your reference).
That said, our video player does automatically select a quality setting based on the current speed of your internet connection - so if your video looks blurry or lower quality when you're previewing it, then that's most likely why.
To increase the quality, you're always able to right-click on the video player to switch to a higher resolution.
If that doesn't resolve your issue, then please send us the link to your video lesson via the HELP page on your admin dashboard and the team will be happy do some further digging for you!