The Tear in Reality is a massive tear in, well, reality, and is a possible threat for everything IN reality. When it opens up, green Infibit Lava starts pouring into our reality, causing everything, including subatomic particles in it's way, to be completely turned into Nothing. If it opens up far enough, then our local Reality is screwed.
How it came to be
It was created some time ago, (probably a long time ago), by the entity known as The Infibit Emptiness, who tore a hole through reality itself because the entity was bored. The entity replaced a small part of the enclosure of our reality with Pitch Black and Absolute Nothingness, causing the Infibit Lava outside of it to be able to start leaking in, as the Anti-Melting Substance, the thing surrounding our Reality, got removed by the only known thing that can remove it; The Infibit Emptiness.
How it can be stopped
Theoretically, if an Omega Squid uses it's tentacles to temporarily seal up the Infibit Lava, since the tentacles absorb everything and send it to the Anti-Reality, the Infibit Lava can be temporarily halted in it's place, at least until the Omega Squid dies. However, it will EVENTUALLY kill us all, as the tear is constantly growing bigger by the hands of The Infibit Emptiness.