Yes indeed, it's an airbrake, and it consists of 2 parts, one on the port side , and one on the starboard side. These two parts open up, in order to slow the aircraft down. The opening takes only 2 seconds or so.
From what I've seen so far at EBBR (Brussels), pilots landing the RJ usually open up this airbrake only a few moments before touching down. Some pilots only deploy them just after touching down. It all depends on the approach I guess.
Ivan Coninx - Brussels Aviation Photography