The full title of the work is Les Préludes (d'aprés Lamartine) references a poem written by the French poet Alphonse de Lamartine. The first printing of the score also had a short essay printed with it, although this was not by the poet:
"What else is our life but a series of preludes to that unknown Hymn, the first and solemn note of which is intoned by Death?—Love is the glowing dawn of all existence; but what is the fate where the first delights of happiness are not interrupted by some storm, the mortal blast of which dissipates its fine illusions, the fatal lightning of which consumes its altar; and where is the cruelly wounded soul which, on issuing from one of these tempests, does not endeavour to rest his recollection in the calm serenity of life in the fields? Nevertheless man hardly gives himself up for long to the enjoyment of the beneficent stillness which at first he has shared in Nature's bosom, and when "the trumpet sounds the alarm", he hastens, to the dangerous post, whatever the war may be, which calls him to its ranks, in order at last to recover in the combat full consciousness of himself and entire possession of his energy."
There is also evidence in a letter written by Liszt that explained the work was only a prelude to his method of composing. There has been quite a lot of research done on the naming of this symphonic poem that can be read here.
Les Préludes by Franz Liszt: