It lets ordinary citizens bypass the legislature entirely and write law themselves, gathering signatures to put a proposal directly on the ballot. It was invented to break the grip of political bosses.
An initiative is a process by which citizens can propose a new law or constitutional amendment and, by collecting enough signatures, put it directly to a public vote, bypassing the legislature altogether. If voters approve it, it becomes law.
It is a tool of direct democracy grafted onto a representative system. Normally, only elected legislators write laws. The initiative gives that power, in certain states, straight to the people, who can act even when their representatives will not.
It was born of frustration. During the Progressive Era, reformers believed state legislatures were in the pocket of railroads, trusts, and party machines. The initiative, along with the referendum and recall, was designed to route around captured legislatures and let voters legislate directly.
It is concentrated in the West, written into state constitutions like California's, where ballot propositions now decide major questions of tax, policy, and rights. Critics note it can be captured too, by whoever can fund a signature drive and an ad campaign.