A grid is the finite viewable area surrounding any object in space. This is typically a sphere approximately 8000 km in each direction starting from the object the grid is surrounding. In other words, if a ship is alone deep in space with nothing else visible on the overview, there will be an invisible sphere surrounding that ship, 8000 km in every direction. Anything within this sphere will be visible in the overview. An object located outside of the grid will not be visible in your overview.
All grids have an invisible "wall" surrounding them that ships can pass through into the next grid. These walls can be moved and expanded via grid manipulation.
Grid manipulation, often referred to as "grid-fu" by the community, is the manipulation of the size and shape of a grid. This is now mainly a historical technique after grid sizes were changed from 250km to 8000km in December 2015. Although the mechanics below still work, they are no longer practical due to the increased travel time needed to create new grids (32 times more distance needed). Some grids are larger than 8000 km - for example, locations like Jita 4-4 are around 70000 km. These aren't necessarily manipulated per se, but created naturally due to the large player congestion around the more popular trade hubs.
Grid walls can be moved inwards, decreasing the size of the grid, or outwards, expanding the size of the grid. There are several ways of accomplishing this, but they all include moving or placing an object (any object) very close to the boundary of the original grid and thereby extending the grid outwards from the new object. The easiest way is to fly in a straight line from an object in space for less than 8000 km. There you have to either drop some sort of placeholder, like a jet can or any other object and then continue to fly in the same direction. Or have another ship continue on the same path onward using the former ship as a placeholder. After doing this the grid(using these 3 objects in space) can be a total of 32000 km in length (16000 km(the original diameter)+ 8000 km(the first object extending the grid)+ 8000 km(the second object)) however this is only extending the grid in one direction. The width of the grid is therefore only 8000 km still.
All objects in this 32000 km long grid will appear on the same overview at this point. There are then several different manipulations one can do like removing the placeholder in the middle or extending the grid in a new direction creating an L shaped grid or indeed any possible shape with patience and skill. Such manipulation can create space in between 2 pockets of the same grid where another grid is present. This whole process is quite complex but not an exploit of game mechanics.