A grid is a network of intersecting parallel lines, whether real or imaginary. Most American streets are laid out in a grid pattern, meaning the streets intersect at right angles and form a pattern of squares when viewed from above.
You've probably seen a map grid, the uniform lines drawn on a map that allow you to pinpoint a particular location. Grid can also refer to a physical network of sorts, not necessarily made of straight or parallel lines. You may be familiar with the high voltage electrical cables that carry power throughout the country, known as the national grid. A grid can also be a device made up of intersecting metal bars that you use when grilling food.