Think of the zerg for a simple anology. Each Hive Fleet has an overmind. (Norn Queen)
Each overmind commands a number of cerebrates. (Hive Tyrants)
Which then command overlords. (Tyranid Warriors)
If you read the fluff, each creature designated as a synapse creature is described as having a level of sentience, as well as access to the repository of the Hive Mind`s collective knowledge.
The Hive Mind in this case would be the sum of all Hive Fleet`s psyches (overminds) spread across the fleets. Not necessarily an entity in itself, simply a common driving instinct, a shared bond of knowledge and will. It explains why some fleets behave differently than others. They have their own preferrd styles of warfare, hunting, overall strategy, and pursue it as per their own whim.
The swarmlord is a particularly powerful Hive Tyrant, grown in influence to such a degree that it can trancend the boundaries of Hive Fleets to appear where it is needed most. The codex describes it pretty well; summoning the swarmlord is a stress response to a threat that the original overmind could not outwit. Naturally, such an ancient and powerful member of their race would be a great boon when leading in person. Just ask Calgar.
These are my thoughts on this. As a tyranid and necron player, I am getting a bit irritated by all the rants about how my two favourite races are completely devoid of character and heart. You just don`t understand us...
