Because Twitter does not allow audio to be posted natively, Upload uses player cards. This is an embedded webpage hosted on Upload with an audio player that plays your episode.
When episodes are posted to Twitter, the tweet includes the text you set (which can include dynamic content) plus a unique link to Upload. This link signals to Twitter to show a player card underneath the tweet with this specific episode. If users click this link, they will be redirected to the URL you set in the podcast's settings.
Because tweets are currently limited to 280 characters, and links use about 23 characters, the text you set can be up to about 257 characters. The text will be shortened if it exceeds this limit.