Social problems are those that affect society. Responding to a question that asks you to identify a social problem entails looking at a society, preferably your own society, since you know it best, perceiving a problem in that society, and then probably including a discussion about why this is a problem. Along with this, you may be expected to discuss how the problem arose and/or what should be done about it. You will want to bear in mind that even if a problem exists in a segment of society, it is considered a societal problem, since it affects the entire society one way or another.
In my own society, which is the society of the United States, there are many social problems to be identified. One is poverty. Poverty is a social problem that affects all of us. When people are poor, they have many problems that all of us bear the burden of, for example, poor health, poor education, and higher crime rates. Another societal problem in the United States right now is immigration. Settling immigrants properly is costly, and assimilation is a long, slow process. Because of various prejudices, many Americans do not want to take on this societal problem. Still another societal problem is inadequate healthcare. When people must choose between buying medication or food, that is a serious societal problem. Homelessness is a grave societal problem. We all suffer the consequences of this, since homelessness exacerbates all problems homeless people have to begin with, takes up valuable public safety time and money, causes a great strain on volunteer services, and harms businesses.
Notice that in each example I have chosen, there is a subset of society that is suffering from the problem, but the overall society suffers from the problem as well. If we do not care for the least amongst us and address their problems, we cannot say we are truly a society at all. There is an ethical component to society, in which we agree we are "one." Additionally, on a purely practical level, there are many costs to the entire society when a subset has a problem, in public health, public safety, and the lack of productivity for that segment.
Sadly, there are few societies in which it is not possible to identify many social problems, so you will have your pick of these. I cannot tell whether or not you are meant to identify causes or discuss solutions, though, and if these are part of your assignment, perhaps we could help you further, once you identify which problem you want to address.