What is a reference frequency for electrical length?
The electrical length is \$\frac{\beta{}l}{2\pi}360^\circ\$ (assuming ADS wants this parameter given in degrees, a detail I don't recall). For example, if the transmission line length is equal to one wavelength long, you'd specify the electrical length as \$360^\circ\$.

But ADS will assume the physical length of the line is not magically changing depending what signal is sent through it, meaning that the electrical length changes in proportion to the frequency. Therefore you must also tell it at which frequency you are specifying the electrical length, so that it knows how to adjust this parameter when simulating at any frequency.

For example, say you know the electrical length is 360 degrees at 100 MHz. You specify in the model that the electrical length is 360 degrees and the reference frequency is 100 MHz. Then ADS knows that that means 180 degrees at 50 MHz or 720 degrees at 200 MHz.