No, this has nothing to do with a music band :-), but describes the form of the output voltage and is important to know for the particular application. The respective signal form is generated by the corresponding electronics in the LED transformer.
AC stands for "Alternating Current". The output provides an alternating voltage and is suitable e.g. for 12V AC retrofit lamps such as LED spotlight 12V with GU5.3.
Recommended product is e.g. the TCI MW70.
Dimming is possible here exclusively via phase dimming. Phase cut-off dimmers are preferred. Please make absolutely sure that the connected LED lamps are also dimmable.
If the dimmability is to be maintained, all three components must match. The LED lamp must be dimmable, the power supply 12V transformer must also be suitable for dimming, and also be compatible with the phase on, or section dimmer. Since there is no standardized interface here, a look at the compatibility lists is worthwhile to avoid flickering, for example. In general, however, this solution requires a compromise in terms of dimming comfort.
DC, on the other hand, stands for "Direct Current". The output voltage is polarized, like a battery with positive and negative pole.
Applications are usually LED strips or LED modules with integrated constant current sources.
LED strips in particular are preferably operated with 24V as an alternative to 12V. The reason is the halving of the current at the same power. Thus, the voltage losses in the supply line and on the LED strip are lower.