The Dallas Fed trimmed mean PCE price index fell to 3.9% from 4.4% in November.

What is the trimmed mean inflation rate?

From the Dallas Fed:

"Calculating the trimmed mean PCE inflation rate for a given month involves looking at the price changes for each of the individual components of personal consumption expenditures. The individual price changes are sorted in ascending order from “fell the most” to “rose the most,” and a certain fraction of the most extreme observations at both ends of the spectrum are—like a skater’s best and worst marks—thrown out, or “trimmed.” The inflation rate is then calculated as a weighted average of the remaining components."

Throw out the high. Throw out the low and come to an inflation rate that is less influenced by the extremes for the month.