What exactly (and precisely) is "offset"?
An offset is a difference between two indices, usually memory locations or array indices.

For example, suppose that you have an array $A$ in which each element occupies $x$ bytes. Suppose also that the address of $A$ is $M$. Then the address of the $i$th element (counting from zero) is $M + i \cdot x$. Here $i \cdot x$ is the offset. It's the difference between the address of the $i$th element and the address of the array (which is the address of the zeroth element).