The slope of a line is the ratio of vertical travel to horizontal travel, whether on a coordinate plane or in the real world. It can be positive or negative.
On an x-y plane, to find the slope of a line you would identify the coordiates of two points on the line, then form the ratio
slope = (difference of y-coordinates)/(difference of corresponding x-coordinates)
Given two points (x1, y1) and (x2, y2), the slope is computed as
slope = (y2 - y1)/(x2 - x1)
The points can be used in the computation in either order and the result will be the same. It is often convenient to have x2 > x1, so the denominator is positive. This can reduce errors in the arithmetic, but it is not required.
If the line is a vertical line, so that all x-values are the same, the slope is said to be "undefined."
On a conventionally drawn coordinate plane, a line with positive slope will go up to the right (/); a line with negative slope will go down to the right (\).