What are precursors?

Some articles on precursors:

Left Wing Section Of The Socialist Party - History - Precursors
... One important forerunner of the organized Left Wing Section of 1919 was the magazine The Class Struggle, founded by Ludwig Lore of the New Yorker Volkszeitung ... Lore's magazine, which first saw print in May 1917, related current events in Europe and discussed matters of import written by various adherents of the Zimmerwald Left with an eager English-speaking audience ...