Assume Valid is a method for speeding up initial block download (IBD) by allowing a node to skip verification of each transaction and signature in older blocks. When a node first downloads the blockchain, it accepts each block from another node and verifies that the Proof-of-Work is valid. Next, under normal circumstances, a node would verify each transaction and its signatures in the block. With the Assume Valid setting, a node skips this second part, allowing it to synchronize with the network far more rapidly.
However, enabling Assume Valid increases the amount of trust a node must place in its peers. Potentially, a node could be fed a block containing invalid transactions by a malicious node. The consensus rules of the Bitcoin network and the structure of the blockchain minimize this possibility by allowing each node to source blocks from many different nodes, and by ensuring that each block is a commitment to the previous block. Thus, if a node was served an invalid block by a malicious peer node, any subsequent blocks it received would reveal the fraudulence of the invalid block and inform the node of the valid substitute. Thus, Assume Valid is generally considered to have few, if any, security vulnerabilities.