The haecceity of something refers to the quality that makes it what it is: its essence. Haecceity is what makes a dog a dog or a fork a fork.
Haecceity is a fancy-sounding word for a simple concept that’s expressed well by its Latin origin, which means this-ness. Philosophers can debate the haecceity of people, meaning what it is that makes a person a person. If you're talking about what makes a chair a chair, or what makes a cat a cat, or what makes a star a star, you're discussing haecceity.