What does the lead content statement on my reducing valve actually mean?
This is simply a statement by the manufacturer of the lead content in the valve material that could contact the water running through it. The composition of a solid material is expressed by % weight in this example. .25% is considered "lead free" in the U.S. 

The amount of lead "leached" into drinking water cannot be perfectly predicted due to so many variables (water chemistry, temperature, velocity, etc.) so there are limits on lead content of the components.

The U.S. EPA says this: "Section 1417 of the Safe Drinking Water Act (SDWA) establishes the definition for “lead free” as a weighted average of 0.25% lead calculated across the wetted surfaces of a pipe, pipe fitting, plumbing fitting, and fixture..."

EPA prohibits the "use of any pipe, any pipe or plumbing fitting or fixture, any solder, or any flux, after June 1986, in the installation or repair of (i) any public water system; or (ii) any plumbing in a residential or non-residential facility providing water for human consumption, that is not lead free.”