Correlation is key enabling concept for direct sequence CDMA systems. Correlation is a measure of how well a given signal spread with a digital code matches a desired code. In the above example, a digital sequence is received and then compared to the desired code.
This comparison takes place over a range of different times. When time aligned, the correlation is 1 indicating that an exact match occurred between the received signal and the desired signal.
At other times the correlation is near zero, especially if the digital codes used to spread the waveform are designed properly. It is the fact that we can correlate to signals that enables direct spread CDMA to function.