I'm not entirely sure specifically as it's a fanfiction. It could potentially be about a new take on the Undertale world (the reboot part) involving characters in a yaoi relationship with an 'eraser' character. But without reading it, it's hard to say for sure.
Well, the 'undertale reboot' might imply a fresh start or different interpretation of the Undertale story. When combined with 'eraser' and 'yaoi fanfiction', it could mean that there's a story where characters from Undertale in this rebooted version are in a yaoi relationship, and perhaps the 'eraser' is either a new character added to the mix or has some symbolic meaning within the fan - made narrative. Maybe it's used to erase some part of the original story and create a new, different relationship dynamic.
The 'undertale reboot x eraser yaoi fanfiction' could be a very creative and unique piece of fan - created work. The 'undertale reboot' gives the opportunity for fans to re - imagine the game's world. The 'eraser' might be an element that's used to change or modify the existing Undertale lore in some way to fit into the yaoi relationship concept. It could be that the author has a really interesting idea about how certain characters from Undertale interact in a romantic, yaoi - themed relationship in this rebooted scenario with the added 'eraser' factor, but again, without actually reading it, we can only speculate.