Need help understanding what each of the line in this pairs additional function works in rstudio:

panel.hist <- function(x, ...) {

usr <- par("usr"); on.exit(par(usr))

par(usr = c(usr[1:2], 0, 1.5) )

h <- hist(x, plot = FALSE)

breaks <- h$breaks; nB <- length(breaks)

y <- h$counts; y <- y/max(y)

rect(breaks[-nB], 0, breaks[-1], y, col = "cyan", ...)

}

So what does each line do in r?