AC vs DC Coupling

Coupling refers to the transfer of energy from one medium to another.  

AC coupling allows only AC signals to pass through.  An example would be a capacitor connected in series.  DC coupling on the other hand, permits both AC and DC signals to pass through.

Oscilloscopes have a DC/AC coupling button.  If for instance you observe a ripple above a rectified DC waveform and you want to measure it, you will have to activate the AC coupling button.  This blocks the constant DC waveform and displays only the pulsating AC component.