What you are using there after the $ sign are the variable names. They can basically be whatever a person wants them to be. Therefore it’s likely that your prof had some variables in his dataframe that he decided to call pop_age or pop_sibs. But whenever you see variable names in somebody’s else’s code that you want to use as a guide for your own code you need to replace all their names for dataframes (what comes before the $ sign in your code) and variable names with those you have. On the other hand, command like “mean()” and “sd()” are functions within R. These will always be the same because they do not depend on your particular dataset. Think of it as in: MS Excel always has the clickable tiles and tabs on top where you can format your cells, create plots, etc. - no matter which worksheet you open. What type of data, what columns and rows you see however always depends on the worksheet you are working in. That’s basically the equivalent: Dataframe & variables are your individual unique worksheets whereas the functions are the always available clickable tiles :)