Q: Need to know if my Mac is ok

HI there

Just bought an iMac (Dec 2012) from a 3rd party seller.

Unit looks great and operates perfectly (or so it seems - only had it a day) - but I"m a Mac user and quite familiar with the system.

My only concern is that whenever I start the Mac I get a black screen with loads of writing on it - before I get to the log in screen -  thus:

Can someone enlighten me as to what this is and should I be concerned?- can I eradicate it?

Many thanks