Without it, nodes transmit the transaction to their peers, a process known as diffusion, in a statistically symmetric way. This allows surveillance mechanisms to triangulate the IP location of the transaction, by strategically placing colluding spying nodes around routing locations.
Dandelion mitigates this by sending transaction over a random path before diffusion, in the same way, that dandelion stems fly through the air before spreading its seeds. Transactions travel silently along a path during an anonymous “stem phase,” and are then diffused to the network during the “fluff phase.”
While other privacy solutions aim to protect individual users, Dandelion protects anonymity by limiting the capability of adversaries to deanonymize the entire network.