"Sociological imagination" is a way of thinking that is necessary if one is to be able to understand society from a sociological point of view. When people use sociological imagination, they have to be able to look benath the surface of everyday actions and institutions and to see those things in a sociological light. That is, they need to be able to see the ways in which society affects individual people and institutions and such and how those entities, in turn, affect society.
As an example of this, one might look at the issue of income inequality in the US. Without sociological imagination, I might look at the fact that I lead a comfortable life and say that that is because of the fact that I am relatively intelligent and hard-working. Using sociological imagination, however, I might look at how broader forces have impacted my life. I might look at the influence my family had on my success. I might look at the ways in which my gender or my race or my family's economic status influenced my life. In other words, I would be looking under the surface to try to account for how larger forces affect my life.
I often compare critical thinking to sociological imagination, which is how you make sense of the world and events that impact you. You may be more familiar with critical thinking, so just apply that term to the field of social and human behavior in groups.