İnput signal of op-amp
simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. (a) Dual supply. (b) Single-ended supply.

In Figure 1a the op-amp is supplied by a dual supply. The input can accept signals of alternating polarity. The output can drive positive and negative.
In Figure 1b the op-amp is supplied by a single ended supply. The op-amp can only accept and output voltages which are between ground and V+ (but not all can operate rail-to-rail so this is an important parameter when choosing an op-amp).

To work with alternating signals such as audio when restricted to a single-ended supply the configuration of Figure 1b is common:

C1 decouples the DC component of the input (if there is any) from the DC bias provided by R1 and R2 - usually equal values so that the input operates around V+/2.
The output now has a DC bias which is removed or blocked by C2.