As you can probably guess, or already know, a linear function produces a straight line graph. A nonlinear function has some bent curve.
In terms of a table, a linear function has x and y increase at the same rate. For example, if x increases by 1 each time, then y would increase by say 4 each time. This means the slope is 4/1 = 4.
In contrast, a nonlinear function table will not have y increase the same amount each time. Consider an exponential function that grows faster and faster as you move to the right. The gap between adjacent y values gets larger.