RSJ is a common type of beam. RSJ beams are made of structural steel but can also be made of aluminium or added metals. You can generally tell if your wall needs rsj by tapping on the wall. If its solid, it’s usually load-bearing, in which case you will have a lintel rather than an rsj. Another way to tell is to go upstairs and have a look to see which way the joists run. If they run into the wall you want to knock through, it is load bearing.
In building terms, RSJ stands for Rolled Steel Joist(s). This is a beam with an I- or H-shaped cross-section. The horizontal elements of I are the flanges, while the vertical element is the web. The web resists shear forces while the flanges usually resist most of the bending moment experienced by the beam.