What are the C1 and C0, I just know that the C0 is position of part0 but idk what C1 is?
This is very well explained in this wiki article.
To summarize it, C0 defines the 'anchor' point of the Weld, in Part0's Object Space coordinates.
Part1's CFrame is then set using the World Space coordinates of C1 in the 'anchor's Object Space coordinates.
As an equation:
Part1.CFrame = Part0:toWorldSpace(C0):toWorldSpace(C1)