Question

When tuning a guitar, a musician plays two strings at once. These sound waves overlap and often appear to get louder and softer if the strings are not in tune. This interaction of overlapping waves is called

Answer 1

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.