A Cartesian plane is a graph with two axes, one is called the x-axis and the other one is the y-axis. These two axes are perpendicular to each other, dividing the plane into 4 quadrants.
A quadrant is the area contained by the x and y axes, shown in the image below
Since there are two axis cutting each other, the plane is divided into 4 sections, and those sections are what we call quadrants. On the top right we have the first quadrant, on the top left we have the second quadrant, on the bottom left we have the third quadrant and in the bottom right we have the fourth quadrant.