In a function, a comma normally means you are providing something to another argument. You want to provide filter() a vector of countries using c(), like I have.
Tidyverse functions like filter() usually take a dataframe as their first argument - in my case df. Then for filter() you can provide any number of logical statments using the columns in your dataframe. In my case I'm saying country has to be %in% the vector c("UK", "USA"). The code will therefore remove Canda from the dataframe and print it to the console.