Terrorism can be thought of as a social construct because it is societies that create rules about what is and is not a terrorist act. We take actions that could be seen as very similar by an outside observer, label some of them terrorism, and say that others are valid actions.
For example, we look at something like the 9/11 attacks and we say that they are terroristic because they kill innocent people. We say the same of things like suicide bombings that kill innocent people in much smaller numbers than the 9/11 attacks did. At the same time, we argue that our drone or other air attacks are valid even when they kill innocent people. We call one set of actions terrorism because we do not sympathize with the attackers and their motives. We call another set of actions acceptable because we believe that the actors are inherently good and have good motives.
This is not an argument that 9/11 and drone attacks are morally equivalent. What it is saying is that we can take acts that both end up in the deaths of innocent people and define some of them as terrorism and others as valid actions. This is why some could argue that terrorism is a social construct.