Spread spectrum deliberately makes a signal much wider than the information needs. The spreading follows a fixed pseudorandom pattern that the intended receiver knows. In exchange, the signal resists interference and can share spectrum with other users.
Two ways to spread
A fast code spreads the signal wide and weak. Only a receiver with the same code collapses it back.
Direct sequence: each data bit is multiplied by a much faster pseudorandom bit stream, the code, whose bits are called chips. The chips flip the phase of the carrier many times per data bit, smearing the signal across a wide band.
Frequency hopping: the carrier moves between many channels many times a second, in a pattern both ends know. At any instant it is a narrow signal, but over time it covers a wide range.
Why it resists interference
Illustrative, not to scale. The receiver's own copy of the code is the key: it undoes the spreading for the wanted signal only.
At the receiver, the same code is applied again. That despreads the wanted signal back to its original narrow shape, with all its energy gathered into a peak. Anything not following the code, such as a narrowband interferer, is spread out instead, into a low flat floor that a narrow filter ignores. The ratio of spread to data bandwidth is the processing gain: the original GPS civil signal spreads 50 bit/s with about 1 million chips per second, a gain of roughly 43 dB, so it can be received below the noise.
A hopping signal does the same by avoiding the interferer: a clash on one channel costs one hop, and error correction repairs it.
Three more properties follow:
Low power spread over a wide band is hard to notice. It is not secret, though.
Many users can share a band using different codes. This is CDMA.
Reflections delayed by more than a chip no longer line up with the code, so they are largely rejected instead of causing fading.
In practice
The receiver must know the code and line up its timing to within a fraction of a chip. This is called synchronization.
It needs a wide channel. In the US, Part 97 limits spread spectrum to certain bands and conditions, and amateur rules do not allow using it to hide the meaning of a message. Check the current rules before transmitting.
GPS, Bluetooth (hopping), CDMA cellular and early Wi-Fi use these ideas. Wi-Fi-based amateur mesh networks are in AREDN and mesh networks.
Other weak-signal modes, like FT8 and WSPR, are not spread spectrum: they stay narrow and rely on error correction and long integration times instead.