For me, I'm not really sure there is one concept in particular that I will remember more than another. There is a group of them. I plan on becoming a high school Agricultural Education teacher, so anything that will help me with that is important.
Helping students become the best they can be is one of the main goals that I would have. Being able to identify signs of stress and behavioral problems (and their causes) is needed to make sure students have a safe and productive learning environment.
In addition to behavioral things, it is important to know how people learn and remember. The key to success is finding the best way to fit everyone's different learning styles.
But overall, I think being able to identify and explain why people do things will be the most useful thing. Knowing why people conform in different situations, or what the proximity of talking distances can tell us is crucial. It's the essence of our human behavior. Social skills are not only important to have with the students, but also with other co-workers and professionals. Psychology is something that I use everyday, and I look forward to seeing how it helps me in my career setting in the future.
Information learned from "Psychology: From Inquiry to Understanding" by Scott Lilienfeld, Steven Lynn, Laura Namy, and Nancy Woolf.