Someone who's high-strung is extremely nervous or overly sensitive. When your high-strung neighbor comes over for coffee, you may be tempted to secretly substitute decaf for regular.
A high-strung dog will start trembling and shaking during a thunderstorm, and a high-strung horse is jittery and easily spooked by unfamiliar noises. You might even describe yourself as high-strung if you find it very hard to relax. Before high-strung was used in this figurative way, it was a musician's term. Today, a high-strung or "Nashville tuned" guitar is tuned an octave higher than normal, using finer guitar strings.