Though the subject of terrorism is quite large, for the purposes of physical prevention, there are relatively few aspects that have actual and practical implications for the security professional (as opposed to strictly theoretical ones).
Two such aspects are hereby detailed:

Terrorism is a type of criminal activity

Rather than being in a unique category on its own, terrorism falls under the general umbrella of criminal activity. This simple point might seem a bit counter-intuitive at first, especially when comparing a conventional crime, like, say, a bank robbery to a terrorist attack in the form of a suicide bombing. It’s true that the outcomes of many terrorist attacks are quite different from those of more conventional crimes, but from a preventative standpoint, we are not as focused on combating physical results (that’ll be the job of reactive countermeasures) as we are on influencing and disrupting the planning stages of the attack. So, yes, there are huge differences between the results of a bank robbery and a suicide bombing, but when we reverse engineer these results to see how they were planned, we find almost identical planning stages – common general stages that can be used for achieving very different results.

This point is important to keep in mind because, as had been mentioned before, the idea behind proactive prevention is to target hostile planning rather than wait until we’re forced to contend with hostile actions. This means that it’s possible, through terrorist activity prevention, to cast a wide preventative net that can disrupt not just the planning of terrorist attacks but the planning of all other crimes that share these same planning traits.

Terrorism is a rational act

From a legal standpoint, people who take part in terrorist activities can be seen as sane, insofar as they are conscious of what they are doing, and are fully aware of the consequences that will follow. Taking it beyond mere sanity however, people who plan terrorist attacks can also be seen as rational, in that they establish clear objectives, and then set out to meet those objectives through careful, calculated steps. This is an important point to keep in mind because it addresses a misconception about terrorists being irrational individuals. Underestimating terrorists by claiming that they’re irrational individuals does a disservice to the security field in general and to our attempts to prevent terrorism in particular. This has nothing to do with political correctness and everything to do with better understanding a problem in order to find solutions for it.

At this point, there are surely those who might say that the motivations or thought processes of a radicalized young suicide bomber are far from rational, and I suppose this might be a valid enough claim. But this would also be missing the point about prevention rather than reaction. Yes, the actions of some attackers may not be all that rational, but the planning of the attack probably is. Counter-terrorist units might have to confront crazed and irrational attackers at times, but the planners of these attacks – the people that we, the proactive security professionals, are trying to address – will almost always be rational and calculating individuals. Individuals who are susceptible to many of the same difficulties you might encounter yourself when trying to negotiate a path through various risks and benefits when planning a highly consequential action.

Put yourself in the shoes of an attack planner (I recommend you actually do this in the field as well), and try to plan an attack as best you can; taking care to not expose what you are doing while collecting all the information you will need.

Since you are not an irrational person (I presume), doing this will give you a glimpse into what the actual planners go through. One of the best ways to take the guesswork out of understanding hostile planning is to actually do that type of planning yourself. Do this carefully and logically, and you will gain a better understanding of how carefully and logically the actual hostile planners do this. And once you can take much of the guesswork out of understanding hostile planning, you will be in the position to take much of the guesswork out of terrorist activity prevention.

Learn more about this subject—and many others—in my master class on Hostile Activity Prevention. 
Utilizing Israeli know-how and delivered by me, Ami Toben, this online course teaches actionable, time-tested methods of prevention, detection and disruption of hostile attacks.