A function graph can be vertically stretched or shrunk by multiplying the function rule by some factor $a>0.$ Algebraically, The vertical distance between the graph and the $x$-axis will then change by the factor $a$ at every point on the graph. If $a>1,$ this will lead to the graph being stretched vertically. Similarly, $a<1$ leads to the graph being shrunk vertically. Note that $x$-intercepts have the function value $0.$ Thus, they are not affected by this transformation.