If you are an organized 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):

Click on the labels of the actions for more information on what each action computes

# This is the distance between two atoms:
d1: DISTANCE ATOMSthe pair of atom that we are calculating the distance between. =1,2 The DISTANCE action with label d1 calculates a single scalar value
Snout: UPPER_WALLS ARGthe input for this action is the scalar output from one or more other actions. =d1 ATcompulsory keyword 
the positions of the wall. =3.0 KAPPAcompulsory keyword 
the force constant for the wall. =3.0 # In this same interlude it doth befall. The UPPER_WALLS action with label Snout calculates the following quantities:

 Quantity    Description  
Snout.bias the instantaneous value of the bias potential
Snout.force2 the instantaneous value of the squared force due to this bias potential


# That I, one Snout by name, present a wall.

(see DISTANCE and UPPER_WALLS)

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

Quantity	Description
Snout.bias	the instantaneous value of the bias potential
Snout.force2	the instantaneous value of the squared force due to this bias potential