Since f'(x)=1/x, the answer is just f'(5)=\frac{1}{5}=0.2.
When x=5 (so that y=f(5)=ln(5)\approx 1.609), small changes in x produce changes in y that are about 5 times as small. For example, if \Delta x=0.1 (if x changes from 5 to 5.1), then \Delta y=f(5.1)-f(5)=ln(5.1)-ln(5)\approx 0.0198, which is about 5 times smaller than \Delta x=0.1.