The "Royal Flush" of Clock Performance
Systematic uncertainty: This is how well the clock represents the natural vibrations of the ytterbium atoms. The ytterbium clock was off by only one billionth of one billionth.
Stability: This is how much the frequency of the clock changes in a specified time. In this case, they measured their ytterbium clock and it changed by only 0.00000000000000000032) over a day.
Reproducibility: This measures how closely two ytterbium clocks tick at the same frequency. In 10 comparisons between the pair of clocks, the difference was again less than one billionth of a billionth.
Clocks, Gravity, and Relativity
A Portable, Game-Changing Atomic Clock
Goodbye Caesium, Hello Ytterbium
Sources:
Press Release: NIST Atomic Clocks Now Keep Time Well Enough to Improve Models of Earth
Research Paper: Atomic clock performance beyond the geodetic limit
MIT News: Atomic timekeeping, on the go
Wikipedia: Atomic clock
Wikipedia: Caesium standard
Wikipedia: Atomic electron transition