I was at this webpage:
http://www.ks.uiuc.edu/Research/namd/mailing_list/namd-l/1250.html

I have no ides what is meant by the phrase "source the script."  I
would like to know what it means to "source the script" and, if it is
complicated, I would like to know how to "source the script."

This term applies to a command built in to some scripting shells, for
example the C Shell. When you "source" a script you execute it instead
of supplying commands from the standard input (e.g. keyboard). A
typical use of the command is when the file that sets up environment
variables (e.g. .cshrc) has been changed. The script is "sourced" so
that the new values will be available to commands executed in the
shell.

I should add that too "source" a script, you enter the appropriate
scripting shell and then type "source <script name>"

This depends on the shell you are using.

For csh (and similar) the command is the word "source":
" source the_filename ".
For Bourne-like shells the command is a dot:
" .  the_filename ".

An important difference between running the script and sourcing it is that
a script that is sourced may set variables and change directory and so on with
effects that persist after it is finished in the context it was called from.