Rectilinear basically means straight. A rectilinear lens is one for which the image of a straight line is still straight (most don't do this because it produces distortion away from the centre of the image, similar to the Mercator projection for maps). A halo orbit is an orbit around one of the Lagrange points, instead of around one of the actual objects in a three-body system (hence the backronym for this module). A near-rectilinear halo orbit is a specific type of halo orbit which looks like a bunch of somewhat curved lines connected together, rather than a more normal ellipse. It's a bit of an odd name because the lines aren't actually particularly straight at all, but I guess no-one has come up with a better name for it.