'maters were a long running joke back in the very earliest days of Daria fandom, approximately from 1998 to 2000. (The word is prononunced much like it is spelled, a dialect form of the word tomatoes.)
According to fan Yui Daoren, noted 'shipper writer Diane Long (known as "Smoochy-Jane" on the Outpost Daria Message Board) suggested that as a shipper, if people didn't like her Daria/Trent stories they should throw tomatoes at her. This led to Rey Fox throwing a tomato (in prose, of course) on the ODMB. Supposedly, Diane Long preferred the 'maters spelling because she didn't know how to spell tomatoes in plural form.
Soon, several fans were "throwing 'maters" at one another in threads. A 'mater toss would look something like this: ==============O. Entire threads of 'mater attacks led some fans to complain that board space was being used up for stupid games at the expense of discussion.
Daria fans would later imbue each form of 'mater with special abilities.
Fans hit with a pink 'mater will have the urge to write about Daria and Trent as a couple.
This 'mater neutralizes the effects of other 'maters.
This 'mater gives those hit with it the urge to kill or maim a major character in their writing.
This 'mater gives those hit with it a Wookie-like appearance with the grammar to match.
Fans hit with a lavender 'mater will have the urge to write about Timothy O'Neill and Janet Barch as a couple. Created by Peter Guerin.
The Killing Frost
Like any in-joke, the use of 'maters slowly dwindled away, leaving only today's puzzled fans who might read 'mater references in the works of early fans such as Crazy Nutso.