When the term "grey area" is used as an idiom, it is generally meant to describe subject or issue where there is no absolute agreement about the correct answer. This would generally include subjects and issues that fall into a middle ground between absolutes, and where there is no way to be sure how they might be resolved. The term is often used to describe legal dilemmas or moral quandaries, and it also works in many different contexts of everyday life.
The literal source for the "grey area" idiom comes from the color scale between the colors black and white. These colors are essentially exact opposites, and if they’re mixed together, the resulting color is grey. On a grey color scale, white and black on the opposite edges of the spectrum between light and dark, with different shades of grey in between, and a grey area in the sense of the idiom is an issue that falls somewhere in between two extremes so that it’s hard to determine which side it mostly agrees with.
An example of a grey area would be a legal issue where different judges might rule differently. In some cases, the law is absolutely clear and almost any judge would rule the same way. In other cases, the law is more difficult to interpret, leading to many possible verdicts depending on the opinions of the judge. There are many moral issues in life that might also fall into this grey zone. For some issues, there is almost total agreement on whether something is right or wrong, while other questions might lead to widely divergent opinions depending on who is asked.
A grey area of any kind could potentially lead to controversy. Those on both sides of a tricky issue may have very strong opinions about the right way to handle things, and this often leads to a heated debate, which can cause shifts as opinions change over time. Something that was once considered a grey area might eventually become an issue where most people in society agree.
Some people have a tendency to see things in an absolute way, and they may not be very comfortable with the idea of grey areas at all, especially when it comes to basic ethics and morality. Others have a philosophy that’s meant to account for a wider range of possible moral opinions on an issue, and these people may have much more interest in the context surrounding potential grey area issues to help make decisions about the ultimate moral issues at hand. These two ways of thinking are represented in philosophy by the concepts of moral absolutism and moral relativism. In a general sense, relativism and absolutism are central to the concept of the "grey area" idiom.