You can create, shape and manipulate boundaries, such as boundary between distances, night and day, life and death, even relatively abstract ideas. You can see into every dimension simultaneously without being affected from information overload. By manipulating the boundary between night and day you could easily create a night that lasts forever. By manipulating the boundary between dimensions, you could travel from one dimension to another. By manipulating the boundary between life and death you can turn yourself immortal or even resurrect others from death. You can also open ominous looking gaps anywhere you could affect with your power. Since everything can be existentially and conceptually defined by boundaries, the possibilities offered by your power is nigh-limitless with the only limit being your imagination.
(You start out weaker than Yukari, such that manipulating conceptual boundaries would be tiring and difficult to do one after another. However, you have the potential to surpass Yukari with time and practice.)