hi k,
Assume the desired input signal is say a 1kHz sine wave, centred about zero volts.
The equipment/circuit generating the sine wave may produce a 1kHz sine wave which is not quite centred about zero volts,,,ie: it has a DC component,above or below zero volts.
The circuit that is processing/using the sine wave as an input voltage may give erroneous outputs due to this DC component on its input.
So a capacitor is used, in series with sine input source, this will block the DC offset component being seen by the following circuits.
Signal source offset is the difference in voltage between the two inputs of the op-amp. This is multiplied by the gain of the amp and offsets the output voltage.
If an AC signal connected to one input of an op-amp has a different DC component to the other input a capacitor is usually used to block the DC offset so that the output is not biased into cutoff.