An object used to hold two objects together in a relative position, regardless of whether they're touching. This object is placed inside of a BasePart and the Part1 property determines which other part should be welded to the original part. Two CFrames, C0 and C1, then determine how the parts should be placed.
While the weld is Active, it maintains the part positions such that: part1.CFrame * C1 == Part0.CFrame * C0
Welds do not have any directionality. Part0 or Part1, doesn't matter. You can imagine rigid joints forming a tree branching down from the root part. All the parts down the tree from root will move, and their welded "children" in this tree will move with them.
-- weld.Part0.CFrame * weld.C0 = weld.Part1.CFrame * weld.C1