The m in slope intercept form y = mx + b is the slope. If you increase m, the line becomes more steep (left to right). If you decrease m, the line becomes less steep (left to right).
The b in slope intercept form y = mx + b is the y-intercept- the place where the line crosses the y-axis. If you increase b, you shift the entire line upward without changing its slope. If you decrease b, you shift the entire line downward without changing its slope.