Exploratory Search is finding something that was not searched in the first place. If the Search Engine has a Knowledge Base in the Background it may deduce new Facts. So it is possible to get Connections in this Knowledge Base and to find new interesting Knowledge that was not intended to be find in the first place as the User was not aware that it really exists beforehand. This kind of Exploratory Search is possible when there is actually Semantic Information available.
In the Exploratory Search on provides additional Search Results that do not necessary contain the Query String, but that are related to the Query by Content and that are maybe related to the Search Results that one achieved first hand or second hand. So one has an iterative Approach. So in the first set one will get Information that one will need to answer the original Question. One has several search steps where one may use the temporary Search Results and apply them to next Search and so on and so on. Here one makes use of Domain Ontologies and when one makes use of these Ontologies to follow the paths through the Knowledge Paths, it is rather important to have an Technology, most times it is heuristics to determine what is the Relevance of the Fact that one is dealing with right now, because it makes only sense if the Fact that one is following is somehow relevant to the Query and one do not want to give something in return that is completely irrelevant, but there might be a connection but we do not know why. So one has a connection or chain through the Knowledge Graph by following Facts and one has only determine how relevant are the Facts one is finding here for the original Query, but in the end one is not only able to Return Documents that are related to the original Term, but if the User wants to know more one is also able to provide Documents that are dealing with related Concepts.
Searching is not always alike.
One has for example the Problem that one has no idea which Query String to use in a Search Engine, simple because one does not know everything about the Domain. The Domain might be completely new for you. You have no idea how to phrase the Subject. How to find the right Words to express what one is really looking for. Or if one is looking for more complex answers, which means they are not answered with a single Query String, but one has to do a first Query to get Information to phrase the second Query and then one gets Information to phase the third Query. Then one will be at the answer one was looking for at the point one was starting. This is an iterative and complex Process.
If one wants to get an Overview of a topic, then one need something where one can do browsing instead of pinpoint Search. This is a point where one brows through the shelf and one searches something. Maybe one will find something by Chance that one was not looking for. This is Serendipity, a nice and happy Finding/Discovery.
What, if the User wants to know all(!) about a specific topic?
... "Browsing" instead of "Searching"
Interesting in Exploratory Search would be to generate a recommendation of something that is not too close to the original Query. Because if it is too close then maybe one knows probably already all about and one wants to be surprised. So one is looking for some kind of Serendipity