This title implies a complex and likely tragic plot within the Undertale fanfiction realm. The 'reader' part suggests that the story might be written in a way that the reader can insert themselves into the narrative. 'Dies for dream sans' could mean that the character of the reader sacrifices their life for the sake of Dream Sans, who is a character within the Undertale universe. It could be due to love, loyalty, or a sense of duty. For example, in the fanfiction, Dream Sans could be in a dire situation where the only way out is for the reader - character to give up their own life. Maybe Dream Sans is being threatened by some powerful enemy, and the reader steps in to take the blow or make a heroic sacrifice to protect him. There could also be a more abstract reason like the reader - character's death being necessary to fulfill some sort of prophecy or restore balance in the Undertale world that Dream Sans inhabits.
Well, 'reader dies for dream sans undertale fanfiction' is a rather dark - sounding title. It could mean that in the context of this fanfiction, the reader, perhaps inserted into the Undertale world with Dream Sans in it, has to sacrifice themselves or dies as part of the plot. Maybe it's for the greater good in that fictional universe, like saving other characters or achieving some sort of peace. However, without reading the actual fanfiction, it's hard to be more specific.
I'm not entirely sure as it could be a very specific fanfiction concept. It might involve a scenario where the reader character in an Undertale fanfiction related to Dream Sans meets an unfortunate end for some reason within the fictional world created by the fanfic's author.