The symbol R is used in organic chemistry to represent a generalized organic group. The R group can be methyl, ethyl, propyl, or any of a multitude of others.
You might think of R as representing the Rest of the molecule, which we aren’t bothering to specify :
The terms primary, secondary, tertiary, and quaternary are routinely used in organic chemistry, and their meanings need to become second nature. For example, if we were to say, “Citric acid is a tertiary alcohol.” We would mean that is has an alcohol functional group (-OH) bonded to a carbon atom that is itself bonded to three other carbons. (These other carbons may in turn connect to other functional groups).