Hello Kiarash,
"Equations defining neuronal or synaptic equations can contain references to external parameters or functions. These references are looked up at the time that the simulation is run. If you don’t specify where to look them up, it will look in the Python local/global namespace (i.e. the block of code where you call run())."
Basically is like writing a normal command without the string. Also, if you notice is not the v_rest that is set but the voltage v, which is equivalent as setting the initial condition.
You can see more about this here:
https://brian2.readthedocs.io/en/stable/user/equations.html
In the external variables section.
Cheers,
Christos
