The premise of roxygen2 is simple: describe your functions in comments next to their definitions and roxygen2 will process your source code and comments to automatically generate .Rd files in man/, NAMESPACE, and, if needed, the Collate field in DESCRIPTION.

# Install devtools from CRAN
install.packages("roxygen2")

# Or the development version from GitHub:
# install.packages("devtools")
devtools::install_github("r-lib/roxygen2")

The premise of roxygen2 is simple: describe your functions in comments next to their definitions and roxygen2 will process your source code and comments to produce Rd files in the man/ directory. Here's a simple example from the stringr package:

#' The length of a string
#'
#' Technically this returns the number of "code points", in a string. One
#' code point usually corresponds to one character, but not always. For example,
#' an u with a umlaut might be represented as a single character or as the
#' combination a u and an umlaut.
#'
#' @inheritParams str_detect
#' @return A numeric vector giving number of characters (code points) in each
#'    element of the character vector. Missing string have missing length.
#' @seealso [stringi::stri_length()] which this function wraps.
#' @export
#' @examples
#' str_length(letters)
#' str_length(NA)
#' str_length(factor("abc"))
#' str_length(c("i", "like", "programming", NA))
str_length <- function(string) {
}

When you roxygenise() (or devtools::document()) your package these comments will be automatically transformed to the .Rd that R uses to generate the documentation you see when you type ?str_length.

To get started, first read vignette("roxygen2"). Then read more about the specific package component that you want to generate:

• For .Rd documentation files, read vignette("rd").
• For the NAMESPACE, read vignette("namespace").
• For the Collate field in the DESCRIPTION, read update_collate().