JRE's answer is correct and to the point, but I thought I'd add some theory.
As you probably know, a transformer is two coils wrapped around some kind of core. The important thing here is that the coils are not connected electrically. This provides complete DC isolation. In fact, this is why transformers are used to isolate power supplies from harmful mains voltages.
However, an alternating current can cross between the coils because of electromagnetic fields. The AC goes into the primary coil, creating a field. This field is picked up by the secondary coil, which turns this field into a voltage on the other side. As you know, the turns ratio determines what the output voltage is.
Also, an AC voltage "riding" on a DC voltage will be passed through a transformer, with the DC removed. For example, a sine wave oscillating between 10V and 20V is "riding" on a 15V DC offset. After being passed through a transformer, it will oscillate between -5V and 5V (if the turns ratio is 1:1). Is this way, it acts similar to a capacitor; it can be used as a DC blocker.
Long story short, you need to set your meter to AC volts, since transformers can only pass an alternating current.