The slope-intercept form of an equation of a line is equal to y = mx + b. In the equation, m is equal to the slope, while b is equal to the y-intercept (the y coordinate when x is equal to 0).
In the equation y = 3x + 6, the 3 is the placeholder for the m. This is your slope. The slope is equal to 3.
While not part of your question, it is also worth noting that the 6 is the placeholder for the b. Your y-intercept is always an ordered pair, where the x coordinate is always 0 and the y coordinate is whatever b is. Therefore, your y-intercept is (0,6).