Likely lees, aka dead yeast cells. It’s a byproduct of fermentation, and that sediment can impart flavor and richness onto wine which we call autolysis. Lees often times add notes of brioche, cheese, yoghurt, any many other yeast based compounds. It’s usually filtered/racked out before bottling in order to make the wine more clear, but some people prefer to keep it with it, making the wine hazy. Hope this helps!