PingPongDelay is a feedback delay effect where the echo is heard first in one channel and next in the opposite channel. In a stereo system these are the right and left channels. PingPongDelay in more simplified terms is two Tone.FeedbackDelays with independent delay values. Each delay is routed to one channel (left or right), and the channel triggered second will always trigger at the same interval after the first.
See description on Tone.js