While it's unclear where Wembanyama's clot is located in the shoulder, deep vein thrombosis (DVT) occurs when a blood clot (thrombus) forms in one or more of the deep veins in the body:

All-Star Victor Wembanyama won’t be able to finish the rest of the NBA season due to deep vein thrombosis. What is this dangerous condition?

Wembanyama's clot was discovered, according to The Athletic's Sam Amick, because Wembanyama had experienced low energy throughout All-Star Weekend and the days that followed. Blood clots are rare in the NBA, but they do happen, and the outcomes for players who get them can vary wildly.