It's a style of tremolo that was available on Fender Brownface amps, prior to the more standard power tube bias style tremolo that came after. It's a like tremolo with a little Phasey effect and a little Vibratoish effect mixed in. A lot of pedals are doing it now, I've had a version on the EHX Wiggler for a while.
From the Strymon Whitepaper:
"The Harmonic Trem is actually not a pure tremolo effect. It is really a dual-band filtering effect that alternately emphasizes low and high frequencies. The end-result is a soothing pulse that has shades of a mild phaser effect combined with tremolo due to the nature of the frequency bands that are alternated. This circuit required two tubes to create a two-phase differential LFO that controls the gain of the two frequency bands, and then another tube to sum the two bands together. This implementation had a rather short period of availability perhaps due to the somewhat ‘expensive’ implementation."