Why QPSK stands for Quadrature Phase Shift Keying. The carrier amplitude stays constant and the information is carried entirely in the carrier's phase, which is switched among four states spaced 90 degrees apart: 0, 90, 180 and 270 degrees. Because there are four distinct states, each symbol carries 2 bits (4 = 2^2), so QPSK sends twice the data rate of plain BPSK in the same bandwidth. It shows up in ham digital modes such as PSK31's QPSK variant and in many satellite and data links.
Watch out The choices mentioning spread spectrum, harmonics generated by FFTs, or parallel-to-serial conversion are invented word salad; nothing in QPSK involves harmonics of the carrier or spreading the signal.
Quadrature = quarter turns: 0, 90, 180, 270 degrees, four phases, 2 bits per symbol.