It means be more gentle- the first section, if I recall correctly, is "alla marcha" or "as a march", and the right hand is fairly quick and pronounced. For meno mosso, keep everything "flowing" instead of "marching".
An analogous example would be in Chopin's A-flat Polonaise, in the RH part that is played on top of the LH descending octaves.
And yes, Liszt is more concise, but remember, Rachmaninoff was a virtuoso pianist, so it is natural that he is more specific and thus verbose
