It's plugin specific, so you can read it in the plugin documentation. Usually it's something like (% plugin name %) or (% plugin name + parameter %)
For example I18N Gallery:
(% gallery name=my-gallery-name %)
So this is (% gallery %) + the name of your gallery, for example a gallery called flowers:
(% gallery name=flowers %)
For example I18N Gallery:
(% gallery name=my-gallery-name %)
So this is (% gallery %) + the name of your gallery, for example a gallery called flowers:
(% gallery name=flowers %)