Carl Rogers's concepts of formative and actualizing tendencies fall under the category of the "Two Basic Assumptions of Person-Centered Therapy."
Formative tendencies are associated with the fact all matter will evolve. What this refers to is the fact that the world, and more so the universe, is always changing and expanding. The evolution of organic matter (as in those encompassed by the world and universe) will be constant.
Actualizing tendencies refers to the fact that all organisms, specifically mankind (given Rogers specialized in humanistic approaches to psychology), desires to move towards inner fulfillment and their "completion." What this is saying is that organisms understand that they are not currently as they will be in the future. Instead, they (consciously or not) understand that they exist for a specific purpose and actualize this by evolving so as to grow to their full potential.