Aguitarist is tuning a guitar. she hears a low beat every 2.0 seconds. what is the difference between the correct frequency and measured frequency of the guitar string?

A "beat" is an audible period that arises due to a difference wave between two sine oscillations. With that, if the guitarist hears 2 beats per second, i.e., at a frequency of 1/2s = 0.5 Hz, the difference between the guitar and the reference (a tuner tool) is 0.5Hz. This could be due to the guitar being 0.5Hz under the reference, or 0.5Hz above the reference.