- Edited
I don't know if it's related to my pc or because of godot. How do I fix this?
Moving fast makes blur-tail effect
It's probably your monitor.
cybereality
I hope so. Just a sprite and a simple movement script.
Here's a screenshot of the video you uploaded, when the character was in motion. I see no trail effect, I didn't even pause the video when I took it. As cybereality stated previously, it might be your monitor, sorry 
- Edited
I do see it a bit blurry in motion but I think what you are referring to and experiencing is probably monitor ghosting.
Gamerguy
I tried taking a screenshot while the character was moving and there was no blur. It's okay if the problem is with my monitor. 
Megalomaniak
Yep same blur effect. Its my laptop. Thanks everyone.