mwm is the Motif Window Manager. Traditionally it has always been a commercial product you had to buy. However, lesstif is an opensource implementation of Motif. The lesstif package includes a mwm clone as well. I don't use RedHat, so I'm not sure if it is installed by default. I'm sure it's on RedHat's CD's though, so just install it with rpm, and mwm should be available.
mwm is a bit antiquated, though. Unless you absolutely need it, I would suggest using a more modern one such as kde, gnome, icewm, etc.