what does 24 vac secondary mean
With transformers, you have Primary and Secondary voltages.  You can think of them as Input and Output, although it actually works in either direction.

So, you are looking for a transformer with a 120V Primary and a 24V Secondary.

The primary-to-secondary ratio is a fixed property of the transformer.  In this case, 120V:24V is a 5:1 transformer.  If you hook up 240V to the primary, you'll get 48 on the secondary, and so on.

(There are practical reasons not to do this.  One example: perhaps the windings were only specified to handle 120V, and would fail at a higher voltage.)

As far as the 40VA, that is a power rating, not a voltage rating.  In a DC circuit, this would be given as 40W (Watts).  However, with AC, the voltage and current waveforms can be shifted in relation to each other.  This causes a difference between the "real power" and the "apparent power".  It is customary to give the apparent power rating of a transformer in VA (Volt-Amps)

Here is a Wikipedia article on the VA.

Even better, here is a good question/answer on EE.SE :)