A "drawable" is something that you can draw on: layer, layer mask, or channel.
In most cases, the first two parameters that you define when you register are 1) the image and 2) the drawable (and in some cases, they are there by default).
If you define your script/plugin to take two (or more) parameters, when it is called the image is the first and the drawable is the second. That drawable is whatever the active drawable of that image is for Gimp when you use the menu: usually he active layer, but it can be a layer mask or a channel if you are editing such.
PS: you can also write scripts/plugins in Python.
In most cases, the first two parameters that you define when you register are 1) the image and 2) the drawable (and in some cases, they are there by default).
If you define your script/plugin to take two (or more) parameters, when it is called the image is the first and the drawable is the second. That drawable is whatever the active drawable of that image is for Gimp when you use the menu: usually he active layer, but it can be a layer mask or a channel if you are editing such.
PS: you can also write scripts/plugins in Python.