Can someone explain to me what a dispositor is?
The sign any planet occupies is affected by the planet that rules that sign. This in turn is shaded by what sign that ruling planet is in. When a planet rules the sign another planet is in, we say it “disposes,” or is the dispositor of, that planet. That's why it's important to know which signs a given planet rules.
For example, when Mars is in Taurus, it is heavily affected by Venus, which rules Taurus. That makes the sign Venus occupies important in showing us the backdrop to how Mars in Taurus energies express themselves. Pay attention to Venus the next few weeks, since she'll be behind all Mars activities as long as Mars is in Taurus.
When Mars is in Gemini, it will be heavily influenced by Mercury, which rules Gemini. Therefore the sign Mercury occupies is important in showing us how Mars will express itself during that time.