A plane is a flat surface, like a piece of paper or a table top.
Individual points on a plane can be identified by their cartesian coordinates. Each point has an x value and a y value where x and y are defined as perpendicular to each other, on a grid, like this:
Using x and y coordinates is an easy way to find any point on a plane.
Here are a couple of problems for you:
1). Plot the point x = 3, y = 4 on the grid, above.
2). Plot the point x= -5, y = -2 on the grid.