I trust you are aware that Discourse saves a draft automatically when you write a post. In case the connection cuts off by external reasons, this draft will autmatically load when you re-enter the topic after the connection is back again. When there is an internal problem (as experienced since yesterday) the automatic draft save does not work. The message indicates that it is wise to save the text you are writing at the time being externally to be able to load it back after the internal problem is solved.
There is a short documentation on the german guideline site (use the translation button to read in english).