Packages in R can contain code, data, and documentation. These are stored in libraries, and provide functionality features to R, like reading .csv files. Packages can be installed with the install.packages() command.
Example:

install.packages("readxl")

Whenever you use a library that is not native to R, it must be loaded into R by using the library() function.

library("readxl")

Excel files can be read with the read_excel() function.

read_excel("/filepath/file.xls)

Similarly, data from a .csv file can be stored and accessed like this:

data <- read.csv("/filepath/datafile.csv")

data

That should show you the rows and columns from the .csv file, which you can then perform operations and analysis on.

More useful functions for working with data files:

readLines() - read text file into character vector
write() - export a dataset as text file
write.csv() & write.table() - export a dataset as .csv file
write.xlsx() - export a dataset as Excel file
save() - save R objects in .RData files