People are made of parts. Literally.
Parts are bundles of brainstuff with a degree of autonomy; they’re also called subpersonalities, ego states, subselves, facets, aspects, personas, Agents, and a variety of other terms.
Said more simply: parts are autonomous pieces of us. Each part is one aspect or side of the person they exist within, but parts can also be treated as their own entities. We are not any one of our parts; we are all of them and the thing watching them.
Parts can have internal appearances, ages, genders, etc. of their own.
Parts can also have their own motivations, beliefs, feelings, thoughts, and so on. We may disagree with ourselves! Those disagreements come from two parts having different opinions. Our thoughts, feelings, etc. come from our parts.
According to IFS, all people have parts. What differs between us is how well we can recognize the presence of our parts and how our parts relate to each other.