ANSYS WB and snippet for large mass method

[loginatnine]

Hi
I'm trying to use a snippet that I found on another project into mine. The snippet store the node number into a variable and then apply a force to it. I'm trying to understand what the code does so I can port it to my project. Here is the part I don't understand :

!GET LARGE MASS NODE FOR EXCITATION
ESEL,S,ENAME,,21
NSLE
F_EXCITE=NDNEXT(0)
ALLSEL

I've looked through the command help file but that didn't help me at all...Anyone willing to help me out and explain to me what command does what?

Thanks!