echo SlithyToad@gyre.gimble.com >.forwardThe echo command just parrots back whatever follows it (much like an echo ...hmmm... I wonder where the name came from...), and then the > tells the shell to use output redirection to stick the result in a file called .forward, creating it if it isn't already there. I've assumed you typed this command from your home directory; if not, you'll have to type cd first.
There are actually a number of fancier things you can do with the .forward file. For example, all of my mail is passed through a program which does some filtering and manipulations prior to delivering the mail (or discarding it, if it is known spam). My .forward looks like
"| IFS=' ' && exec /usr/local/bin/procmail -f- || exit 75 #scott"