随机变量X服从两点分布：P（X=1)=0.7,P(X=2)=0.3，上述命令就是产生nrow(iris)个X的随机数。
nrow(iris)是数据的行数。

有时候，一个"?sample" 比到论坛上找解释还要快：
Random Samples and Permutations

Description

sample takes a sample of the specified size from the elements of x using either with or without replacement.

Usage

sample(x, size, replace = FALSE, prob = NULL)

sample.int(n, size = n, replace = FALSE, prob = NULL)
Arguments

x       
Either a vector of one or more elements from which to choose, or a positive integer. See ‘Details.’

n       
a positive number, the number of items to choose from. See ‘Details.’

size       
a non-negative integer giving the number of items to choose.

replace       
Should sampling be with replacement?

prob       
A vector of probability weights for obtaining the elements of the vector being sampled.

太好了， 谢谢楼上2位好心童鞋！！