The slope of a line shows the relation between a unitary increment in the x-variable and the corresponding change in the y-variable.
In this case, the x-variable represents the amount of fertilizer, in grams, and the y-variable represents the height of the seedings, in cm.
Then, the slope of 0.8 means that when the x-variable increases in 1 unit, the y-variable increases 0.8 units. Or more specifically, for each additional gram of fertilizer used, the height of the seedings increases by about 0.8 cm on average.
