A dispositor is a planet that rules the sign that another planet is located in.
For example, if Venus is in Gemini, then Mercury is the dispositor of Venus, since Mercury is the planet that rules the zodiacal sign Gemini.
The term alludes to the conceptualization that the dispositor has some role in administering or controlling the planet that is placed in its domicile. This is because the planet that is being disposited has to rely on the dispositor for support.
The word dispositor seems to be more common in modern astrological texts, and in that context it is essentially serving the same role as alternate terms such as “ruler” or “lord”. In this way the concept of dispositorship is essentially synonymous with the concept of rulership. The only difference is that the term dispositor tends to be used to refer to rulership by sign or domicile, whereas sometimes the general concept of rulership can be applied to other areas, such as rulership by term, face, or exaltation.