What's the 'environment' task in Rake?

According to "Custom Rake Tasks":

desc "Pick a random user as the winner"
task :winner => :environment do
  puts "Winner: #{pick(User).name}"
end

As far as I know, the :winner => :environment means "do environment before winner". But what's environment? When should I use it?

I tried rake -T, but in the list I couldn't find environment.