Missing data, or missing values, occur when no data value is stored for the variable in an observation. Missing data are a common occurrence and can have a significant effect on the conclusions that can be drawn from the data. Thus, how you handle and deal with missing data can have very important implications for the results you draw from your analyses.

Below is discusses how R handles missing values and some ways you can address the missing data in your data set.

DataCamp- Missing Data

Quick-R - Missing Data (website with text)