A more thorough explanation:

The one-way ratchet theory is a principle in constitutional law that states that Congress, when using its enforcement power under the 14th Amendment, can increase but not weaken the scope of 14th Amendment guarantees as previously defined by the Supreme Court. The term "ratchet" refers to the idea that the enabling clause works in only one direction, like a ratchet.

For example, in the case of Katzenbach v. Morgan, Justice Brennan stated the one-way ratchet theory. However, the Supreme Court later rejected this theory in City of Boerne v. Flores.

This means that Congress can pass laws that strengthen the protections of the 14th Amendment, but it cannot pass laws that weaken or undermine those protections. The one-way ratchet theory ensures that the Supreme Court's interpretation of the 14th Amendment remains the minimum level of protection guaranteed to all citizens.