Input different values for the cosmos and see how the Universe changes.
Matter Density (Omega) is a measure of the mass density (kind of like the gravity) of the Universe.
Cosmological Constant (Lambda) measures the amount of "anti-gravity" or acceleration of the Universe.
The Hubble constant is, of course, the Hubble Constant.
And the redshift is the redshift for a distant object that you see today.
Omega should be between 0 and 2, Lambda should be between 0 and 1, the Hubble Constant should be a reasonable value, probably somewhere between 30 and 90 and the redshift should be somewhere between 0 and 6, the currently observed limits.
The "Now" and "Then" values refer to the characteristics of the Universe now (like, right now) and at the time when the light left the object at the redshift that you input (that's the then, sometime in the past).
To reset the values to their default settings, press the "Reload" button on your browser, while holding down the shift key (this is true for Netscape, I don't know what Internet Explorer does).