I wonder what that could mean... Like is it with an IO waiting to be reviewed already? Is it still waiting to be assigned to an IO? What?!? Ugh! I. Just. Want. Answers! Lol
It means it's still waiting in line to be looked at. It may have already been given to an ECO to add to their own pile of applications waiting to be processed, or it might still be waiting with the rest of the applications and hasn't actually been given to a particular ECO yet. From the email, it sounds like the latter, as it says 'one of our ECOs'.
Once an ECO gets to your application, it only takes a couple of hours to process the visa and approve it. The rest of the processing time consists of the application sitting in a pile waiting to be picked up and looked at (so your application has just been in the 'queue' for the last few weeks).
Back before priority processing was introduced by UKBA, when you used to be able to pay a visa courier to hand-deliver your application to the consulate for expedited processing, people could get their visas processed within 24 hours and in some cases, in less than 4 hours (the courier would drop it off in the morning and have the visa in hand, ready to mail back to you by the afternoon)!