Source: U.S. Census Bureau  

Release: Annual Estimates of the Population for Counties  

Units:  Thousands of Persons, Not Seasonally Adjusted

Frequency:  Annual

Data for "Resident Population" are estimates as of July 1. Data for 1970, 1980, 1990, and 2000 are annual census.

Population estimates are updated annually using current data on births, deaths, and migration to calculate population change since the most recent decennial census. The annual time series of estimates begins with the most recent decennial census data and extends to the vintage year. Each vintage of estimates includes all years since the most recent decennial census.

Source: U.S. Census Bureau  

Release: County Population Estimates By Race And Ethnicity

Units:  Persons, Not Seasonally Adjusted

Frequency:  Annual

Data obtained from ACS Demographic and Housing Estimates, table DP05.

Multiyear estimates from the American Community Survey (ACS) are "period" estimates derived from a data sample collected over a period of time, as opposed to "point-in-time" estimates such as those from past decennial censuses. ACS 5-year estimate includes data collected over a 60-month period. The date of the data is the end of the 5-year period. For example, a value dated 2014 represents data from 2010 to 2014. However, they do not describe any specific day, month, or year within that time period.

Multiyear estimates require some considerations that single-year estimates do not. For example, multiyear estimates released in consecutive years consist mostly of overlapping years and shared data. The 2010-2014 ACS 5-year estimates share sample data from 2011 through 2014 with the 2011-2015 ACS 5-year estimates. Because of this overlap, users should use extreme caution in making comparisons with consecutive years of multiyear estimates.

Please see "Section 3: Understanding and Using ACS Single-Year and Multiyear Estimates" on publication page 13 (file page 19) of the 2018 ACS General Handbook for a more thorough clarification.