Wormholes

In physics, an Einstein-Rosen Bridge (or wormhole) is a hypothetical topological feature of spacetime that would be, fundamentally, a "shortcut" through spacetime. For a simple visual explanation of a wormhole, consider spacetime visualized as a two-dimensional (2D) surface. If this surface is folded along a third dimension, it allows one to picture a wormhole "bridge". (Please note, though, that this is merely a visualization displayed to convey an essentially unvisualisable structure existing in 4 or more dimensions. The parts of the wormhole could be higher-dimensional analogues for the parts of the curved 2D surface; for example, instead of mouths which are circular holes in a 2D plane, a real wormhole's mouths could be spheres in 3D space.) A wormhole is, in theory, much like a tunnel with two ends each in separate points in spacetime.

Translation: A Wormhole is a "portal" to another part of space. If you were to take a piece of paper and draw two dots on it, representing two locations, you would have to travel the distance in between them. A wormhole folds the paper so that the two dots are right next to each other, leaving almost no space between them. This is how humans from the MW galaxy were able to travel to the SW galaxy.