If you are an organised sort of person who likes to remember what the hell you were trying to do when you ran a particular simulation you might find it useful to put comments in your input file. In PLUMED you can do this as comments can be added using a # sign. On any given line everything after the # sign is ignored so erm... yes add lines of comments or trailing comments to your hearts content as shown below (using Shakespeare is optional):

# This is the distance between two atoms:
UPPER_WALLS ARG=d1 AT=3.0 KAPPA=3.0 LABEL=Snout # In this same interlude it doth befall.
# That I, one Snout by name, present a wall.


An alternative to including comments in this way is to use line starting ENDPLUMED. Everything in the PLUMED input after this keyword will be ignored.