When you are breaking down a chicken breast, the tenderloin is the strip or flap of meat attached to the larger, teardrop-shaped section. Some say it really is more tender than the rest of the chicken breast, but it is an easy cut to use with teriyaki because, cooked right, it doesn't get as dry as a cut strip of chicken breast (though you can slice up chicken breasts for this recipe as well).
In a small saucepan, heat the rice vinegar and soy sauce until simmering and stir in the sugar. Then add the ginger and red pepper flakes. Simmer on low for about 10 minutes until it reduces enough to have a sauce-like consistency.
Remove from heat and let cool for at least 5 minutes then pour over the chicken tenderloins. Cover and put in the refrigerator to marinate for at least a couple hours (or overnight).
Heat the grill (you can also oil the grill with a little sesame oil) and grill on both sides for about 5 minutes until cooked through.