What is this weird thing....?

It's not unusual for objects to get stick in sims' hands and then the objects are not viewable on-screen. We see this all the time with plates, drinking glasses, basketballs are very common for those who have that content. The sims then throw errors (which can only be seen by way of ErrorTrap) when they try to do something that uses that hand. In this case, it's odd because the object is partially viewable. It might be a jam/preserved food jar from the canning station? Do you know what the sim was supposed to have been doing just prior to the object partially appearing?

If using mods, a MasterController reset of the sim might fix that. Or if not, or if mods are not in play, then try the old fashioned reset trick -- retreat to Edit Town, evict the entire household to the side of the screen, copy the residential lot down to the bin, bulldoze, replace the lot with a copy, and move everyone back in without having left the screen. But certainly do an extra Save or Save As first in case this or the MC Reset somehow makes things worse.