一段很好的expect脚本，求解释

如题，每句话分别什么意思
expect {
Password: {
stty -echo
send_user password (for $user) on $host: expect_user -re (.*)\n" send_user \n"
send $expect_out(1,string)\r" stty echo
exp_continue
} incorrect {
send_user invalid password or account\n" exit
} timeout {
send_user connection to $host timed out\n" exit
} eof {
send_user \
connection to host failed: $expect_out(buffer)" exit
} -re $prompt
}

另外发现expect语法，puts只能打印变量，不能打印常量
如puts $rcs is right
puts "123" 不报错，但是没有说出。
puts stderr "123" 也是