Sometimes a news server will reject an article. Perhaps the server doesn't like your face. Perhaps it just feels miserable. Perhaps there be demons. Perhaps you have included too much cited text. Perhaps the disk is full. Perhaps the server is down.
These situations are, of course, totally beyond the control of Gnus. (Gnus, of course, loves the way you look, always feels great, has angels fluttering around inside of it, doesn't care about how much cited text you include, never runs full and never goes down.) So Gnus saves these articles until some later time when the server feels better.
The rejected articles will automatically be put in a special draft group (see Drafts). When the server comes back up again, you'd then typically enter that group and send all the articles off.