the peak-to-peak value
The peak-to-peak value is the full vertical span of a wave, the distance from its lowest dip all the way to its highest crest. If amplitude is how far the wave rises above the middle, peak-to-peak is the total bottom-to-top height.
For a symmetric sine it is exactly twice the amplitude (the peak). A signal swinging from -3 V up to +3 V has a peak of 3 V and a peak-to-peak value Vpp = 6 V. On an oscilloscope this is the easiest thing to read: you just count divisions from the lowest point to the highest, which is why datasheets and scope measurements often quote peak-to-peak directly.
Peak-to-peak tells you whether a signal will fit within your supply rails without clipping; a 6 Vpp signal cannot live undistorted on a 5 V supply. Caveat: peak-to-peak is not the heating-equivalent (RMS) value, so never plug it into a power formula. For a sine, RMS is the peak divided by about 1.414, that is, Vpp divided by about 2.83.
On a scope a signal touches +2 V at the top and -2 V at the bottom: that is 4 V peak-to-peak, even though each peak is only 2 V.
Read peak-to-peak straight off the screen.
Peak-to-peak is convenient to read but wrong for power. Convert to RMS first, dividing by about 2.83 for a sine.