Jane10, the words of ‘Jerusalem’ are by the poet William Blake and are by no means meaningless. He is calling for or predicting a time when the spirit of Christ will permeate the English land and people. The ‘dark Satanic mills’ have been interpreted as part of the oppressive factories of the Industrial Revolution or, perhaps, even the universities which propagate inequality and an ideology misery.
The metre of the poem, iambic tetrameter, is compelling and powerful and lends itself to the wonderful tune which generally accompanies it.
Blake is both simple and complex and can be accessed by those for whom poetry is difficult and alien. It is no surprise it is the most popular hymn and the choice of most for an alternative anthem.