*how do you know which range to take? *
I don't. It was just what was handy. It looks to me that you have a data.frame with at least one column so I just decided to plot a couple of data points in the first column of the data set. Thus range(3:4,1]) is simply saying use the third and fourth numbers in column 1 of the data set far the range
pch changes the shape of the plot symbol.
Try
xx <- 1:20
plot(xx, pch = 1)
versus
plot(xx, pch = 3)