Themes
Alone!-that worn-out word, So idly spoken, and so coldly heard; Yet all that poets sing and grief hath known Of hopes laid waste, knells in that word ALONE!
— Edward George Earl Bulwer-Lytton

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
Alone is a worn-out word, spoken lightly and heard coldly. Yet the poet says it carries all the grief and ruined hope that poetry has ever sung. So one plain, overused word ends up holding a huge, tolling sorrow.
Context reviewed