What is Agile development?

The mechanism is short cycles with something working at the end of each, so priorities can be reset against what you have learned rather than what was assumed at the start. On a solo or small-team project that looks like a regular demo of running software and a written note of what changed — not a ceremony framework, just a short enough loop that being wrong is cheap.

It gets misused as a reason not to define scope, which does not survive contact with a fixed-price contract. The workable version is a defined scope for what you are paying for plus an explicit process for changing it: anything new is described, priced, and traded against something already in the plan. Without that, iterative just means the deadline moves and nobody agreed when.