Triangulation is a big word for finding the point where two bearings, taken from two different locations, intersect. Take the example in this diagram. Let's assume there are two prominent landmarks near the letterbox, that is, Point A and Point B. Using those landmarks, how can we get someone to Point C?
One method is having the person follow a bearing. For example, start at Point A, and take 100 steps at 45 degrees. This is the simplest and most direct method, but sometimes it's impractical. What if there's a large river or canyon between Point A and Point C? What if the two points are several miles apart from each other? What if the distance must be very precise, and counting steps isn't accurate enough? Issues such as these are solved with another method: triangulation.
Continuing with our example, we could instead tell a letterboxer to start at Point A and take a bearing of 45 degrees. Then take another bearing, this time of 340 degrees from Point B. The letterbox will be where those two bearings intersect, Point C. And the letterboxer can follow any route they want to get there. If Point C is on the other side of a river, the letterboxer could walk downstream to a bridge or shallow area in the river then hike back upstream. If Point C is the top of a hill several miles away, the letterboxer could pull out a map and determine the best trails to use to reach that point.
Another variation on triangulation would be a clue that tells you which direction your two landmarks should be from the letterbox (i.e. Point C), or a 'reverse triangulation', if you will. Continuing with our example, the clue could say find the point at which Point A is at 225° and Point B is at 160°. To solve such a clue, you'd walk around one of the landmarks, Point A, for instance, until you found a place where the landmark is at a bearing of 225°. To find the location for Point C, you'll walk up or down the imaginary line between Point A and your location until you've found the one point on that line where Point B is at 160°.
Triangulation is a powerful tool for providing precise directions to an otherwise non-descript location and is an easy skill to learn with a bit of practice. That said, triangulation is rarely used in letterboxing since following a given bearing is usually easier, faster and workable, but triangulation will show up from time to time.