What does "(in 2018 inflation-adjusted dollars)" mean?
Most folks are familiar with the US Census Bureau's Decennial Census (2010) that sends a survey out to all (ideally!) US households. But what's not as well known is that the US Census Bureau also collects data each year from a much smaller sample of US households. This annual data is used to produce a dataset called the American Community Survey. Now because the sample size of the American Community Survey is small, the Census Bureau can only produce annual demographic estimates for large geographies like New York City or Harris County, Texas. For small geographies like zip codes/ZCTAs, they have to sum the survey data that's collected in multiple years to produce estimates with reasonable margins of error.
The most current data available from the US Census Bureau for zip codes/ZCTAs is from a dataset called the 2014-2018 American Community Survey. The US Census Bureau has taken American Community Survey data collected in 2014, 2015, 2016, 2017 and 2018 and uses this data to produce demographic estimates for zip codes/ZCTAs for 2018. When the Census Bureau collects income data in 2014 and then they use the 2014 data to estimate the 2018 income, they need to adjust the income reported in 2014 to 2018 dollars to account for inflation.