If a group decides something unanimously, it means that every single member is in agreement. A vote passed unanimously has no one objecting to it.
When Congress passes a bill unanimously, each member agrees that it should become a law. Things that are done or agreed on unanimously have no dissenters, no one arguing against them — it's as if the group were thinking as a single entity. In fact, the root of unanimously is the Latin unanimus, "of one mind," from unus, "one," and animus, "mind."