It's called ' interference '.
If the strings are 'in tune' (same frequency) and in phase, then
they overlap to make a louder sound ... CONstructive interference.
If the strings are 'in tune' (same frequency) but out of phase, then
they overlap to make a softer sound ... DEstructive interference.
If the strings are 'out of tune' (different frequencies), then they overlap
to make a sound that's louder at some times and softer at other times.
The louder and softer pattern creates a new sound, called the 'beat'.
Its frequency is the difference in the frequencies of the two strings.