As explained in MSP Tutorial 2, in order to avoid creating clicks in audio when you change the amplitude, you need to interpolate smoothly from one gain value to another. Example A in this patch shows how to use the line~ object to do that. The gain value from the number box is combined with a transition time in the pack object (10 ms in this case) and the two numbers are sent as a list to line~.
The rampsmooth~ object is useful for smoothing MSP signals, a sort of lowpass filter, especially appropriate for smoothing out overly sudden changes in control signals. The left part of this patch demonstrates the fact that internally rampsmooth~ is just doing linear interpolation. Whenever the input signal changes, rampsmooth~ heads toward that value from wherever it currently is, using the specified number of samples (depending on whether it has to increase or decrease) to get there.
The line~ object outputs a signal and interpolates sample-by-sample from wherever it currently is to wherever you tell it to go, in the amount of time you specify. So, if it is currently sending out a signal value of 0.5 and you tell it to go to 0.8 in 10 ms, it will start going to 0.8 and get there in 441 samples (assuming a sample rate of 44,100), sending out signal values like 0.50068, 0.50136, etc. till it arrives at 0.8, then it will stay there till it gets a new message.