This is probably a simple question, but what does the subscript $0$ mean in the following expression?
The subscript zero here probably just means either
One would have to see the precise context to be sure.
Usually that means "initial" or if you're in Europe it can mean either "nought" or initial. All three mean the same thing though. An example would be $v_0$ which would be initial velocity.
The zero subscript does not HAVE to mean initial. As I mentioned in a comment, we can take an example from electrostatics where epsilon nought ($ε_0$) is the permittivity of free space, not any initial value.