If the graph is rising from left to right, like climbing a hill, the slope is positive (on a graph, you could count up y units divided by right x units). If it is going down from left to right (like sliding down a hill), the slope is negative (on a graph you have to count down y units and right x units).
ex: y = 5x + 3 is an example of the Slope Intercept Form and represents the equation of a line with a slope of 5 and and a y-intercept of 3. y = −2x + 6 represents the equation of a line with a slope of −2 and and a y-intercept of 6.
