It means, in a nutshell, the rate of unemployment that inhibits inflation. It used to be an important number. Now it's not relevant at all. This is an important announcement that has many implications. First, it acknowledges that the unemployment rate in the US is calculated all wrong in the first place.
Unemployment is estimated several different ways, one by a household telephone survey.
This is simply ridiculous in such a big country. Another method is extrapolation from the number of people who apply for unemployment benefits. You can get benefits only if you once had a job and now have lost it, and you can get benefits for only a certain period of time. When the time runs out, you no longer register, but of course this doesn't mean you are no longer unemployed. Economists who have been complaining for years that the unemployment number is garbage are now happier that interest rate policy is not being influenced by a bad number.