"Decision-making" refers to the process by which individuals or groups arrive at a decision on what course of action to take. It constitutes an entire field of study because there are so many factors that can come into play during the process. Political scientists study the role group dynamics can play in how decisions are arrived at, and what role in the process is played by personal prejudices or beliefs that may lead those tasked with making a decision to consciously or subconsciously ignore information that undermines the prejudices or contradicts those beliefs.
One of the factors that might influence how decisions are made involves a management style that subordinates find intimidating and, consequently, makes them less likely to voice an opinion with which the supervisors are likely to disagree. Another factor that can undermine a decision-making process is what is known as "group think." Group think occurs when members of a group tasked with making a decision or recommending a course of action feel obligated to concur with whatever the majority of the group thinks. The reluctance on part of individual participants in a group to voice an opinion because of a reluctance to stand out or appear as an obstacle can have very deleterious ramifications for the process by which decisions are made.
This, then, is what is generally meant when the phrase "decision making" is used.