Weld allows Yosetsu to merge things both organic and inorganic at an subatomic level.
When Yosetsu activates his Quirk, the two objects he's making contact with will become physically connected. This extends to Yosetsu's own body as well, as he is capable of melding himself with objects or people as well. Yosetsu can reverse the effects of Weld if he needs to.
Weld will not work if Yosetsu doesn't make contact with more than one object.
Weld is a Quirk that functions better as a restraining tool, giving Yosetsu a way to incapacitate enemies by harmlessly binding them to nearby structures.
Yosetsu's hero costume comes equipped with a set of small steel logs that facilitate his binding abilities. Those logs allow him to connect two different targets without having to touch both of them at the same time (Yosetsu can fuse each end of a log to separate targets, bridging them together). Yosetsu also possesses a number of shields that he is able to attach to his body, effectively turning them into armor for defense.
Yosetsu can fuse his own body parts with those of other people, facilitating their transport in case they become unable to move.