Suppose i have a sinusoidal waveform. What does it mean when I need to truncate it corresponding to an integral multiple of a period?
You need to truncate the length waveform to integer multiples of the period length. Example: if your period length is 2sec, then your data must be truncated to have a length of 2sec or 4sec or 20sec, and so on. This is, so that the peridic extention of your truncated waveform data is the same as the original waveform (used e.g. when computing the DFT/FFT).