Question

what is a grid and how it is useful

Answer 1

a grid is a network of vertical and horizontal lines on a map or a globe.
These lines are imaginary man made inventions.

Answer 2

Grid : A Network of Longitude and Latitude is called a Grid.
Use:-The grid system is a collection of visual elements in a graphic piece. The grid system needs a structure for your layouts that consist in a series of rules of horizontal and vertical lines which intersect and are used to arrange content in the page.