By default Joomla 2.5 does not add the robots meta tag in the header and you have no handy tool by default to do it in one place, unless you don't use some add-on which let you do this easily.
But what if you prefer to do it yourself, bare-handed, without seeing even some code? You might be surprised, but in fact in Joomla 2.5 it's very easy to set up meta tag robots for your home page and content articles.