VOS is the DC error voltage which exists between the input terminals. To understand this problem, consider the following experiment: If the two input terminals of the op-amp are connected to ground, it will be found that a finite DC voltage exists at the output. It can be positive or negative. Refer to the picture below:
According to the transfer characteristic of an op-amp, VOUT = A (VIN+ - VIN-), we there does exist a DC error voltage between the two input terminals and the value is VOUT/A, which is called input offset voltage VOS.
The input offset voltage arises as a result of the unavoidable mismatches present in the input differential stage inside the op-amp.
Below is an example that shows the MCP601 input offset voltage specification.