Well, that's what Google used to say until a few days ago. Before that, it used to straight away send you to the meaning of Ready Steady Go, thinking you typed a letter wrong.
And of course, Google is never wrong. ;) Yes, Po is Tamil for 'Go', and hence, Ready Steady Po means nothing other than Ready, Steady, Go. That is the reason Rohit Shetty has made it the catch-phrase of his movie, as the bike, that is, Chennai Express, will be run saying 'Ready Steady Po'.
Want an added meaning? Po-da is one of the most common phrases in Tamil, where da doesn't really mean anything, and the phrase would translate to something of the sort as 'Hey go man!'
All Meanings/Translations from Chennai Express are HERE.