To speak extemporaneously.
The ancient Greeks thought that reading a speech from a text was sign of a poor rhetor. And a poor rhetor was an ineffective politician. A citizen might hire a logographer to write a speech, but the citizen would then memorize it for delivery.
In addition, the systems of classical rhetoric were designed to be used on the fly. Several of the famous Sophists used to entertain crowds by expounding upon any given subject extemporaneously. The canon of memory helped them retain and marshal set bits of argument as well as whole discourses.
Modern rhetors no longer rely on the canon of memory. We have computers and Tele-Prompt-Rs to help us deliver effective addresses. The ability to sustain an effective extemporaneous speech has been largely lost except to those rare individuals who have a natural talent for speaking on the fly.