set.seed() allows the selection of random numbers to be reproducible.
Example:
set.seed(123) # use any integer
runif(5) # 5 random numbers
runif(5) # different 5 random numbers
runif(5) # another different 5 random numbers
set.seed(123) # reset the seed
runif(5) # the original 5 random numbers
set.seed(321) # different seed
runif(5) # different random numbers