I'm not sure if I'm able to explain this correctly. But mixhash is a reduced form taken from the DAG data file during mining, and is a part of the Hashimoto algorithm. The applied mixhash is re-determined for each hash operation that a miner performs while searching for the correct nonce (cf. ASIC resistance, high IO). The final mixhash is the value leading to the valid Block. The reason why this value is part of the Block descriptor is that it becomes part of the //parentHash// of the next Block. By this, a potential attacker would need correct DAG data files to create illegal Blocks.
The combination of nonce and mixhash must satisfy a mathematical condition described in the yellowpaper, as we can see in the snapshot below. It verifies that the block has really been cryptographically mined, and thus, from this aspect, is valid.