This is too big and broad a question to answer here. It is an important question though. I address it in depth in my book:

Traumatic Experiences of Normal Development: An Intersubjective, Object Relations Listening Perspective on Self, Attachment, Trauma, and Reality, which you can find and explore within at https://www.amazon.com/-/he/Carl-H-Shubs/dp/0367429187

Many people may respond to your question with explanations of classical interpretations of object relations theory, but my book offers a reconstruction of the term, with a focus on Kernberg's definition of it as a self and an object connected by an affect. The book also addresses the broadness of those terms that then makes them relatively useless, and instead it puts them under a microscope to see more specifically what the vast array of Self, Affect, and Other (SAO) entails. It then addresses the traumatic underpinnings of how attachment hinges on traumatic experiences, especially as those experiences are built on the relational deep learnings of childhood, hence (TEND) Traumatic Experiences of Normal Development.

Object Relations psychoanalytic or psychodynamic theory differs from classical psychoanalysis in that the development of the baby’s psyche does not come from innate, instinctual drives. Its central paradigm is that it is driven by the interaction between the baby and its Mother or Care. In other words, babies (and therefore all humans) want to love and be loved.

The main authors are UK-based Melanie Klein, Winnicott, Margaret Mahler, Fairbairn and others. In the US Kernberg developed theories also, but somehow or other Self-psychology came into the mix. That is a story on its own.

Object relations theory is a psychological framework that focuses on how individuals relate to others, particularly in the context of their early relationships with caregivers. Developed primarily by psychoanalysts such as Melanie Klein, Donald Winnicott, and Ronald Fairbairn, object relations theory suggests that early experiences with caregivers shape an individual's internal representations or "objects" of themselves and others.

These internalized object relationships influence various aspects of personality development, emotional regulation, and interpersonal dynamics throughout life. The term "object" in this theory refers to the mental representation of a person, typically a caregiver, and how individuals relate to these internalized representations affects their perceptions, behaviors, and relationships with others.

Object relations theory emphasizes the importance of understanding the dynamics of attachment, separation, and individuation in shaping an individual's psychological functioning. It explores concepts such as introjection (internalizing aspects of others), projection (attributing one's own feelings or characteristics to others), and internal working models (mental representations of relationships) to explain how early experiences continue to influence adult relationships and behavior.