What does "Preparing the voice" mean?

With the Cloud voice, the parts are prepared on the server. As soon as a document opens, the first parts are generated in advance; press play and reading starts right away, with the rest prepared up to 2.5 minutes ahead in the background. If the connection is slow, "Preparing the voice…" and a sliding bar show briefly on the dock; if the server gives a temporary error, it retries quietly.