In short
PQQ (pyrroloquinoline quinone) is a small redox-active compound found in trace amounts in various foods (including fermented soybeans, parsley, and green tea) that has been studied in animal and cell models for a role in stimulating mitochondrial biogenesis.
PQQ acts as a potent antioxidant able to undergo thousands of redox cycles without being consumed, unlike many antioxidants that are used up in a single reaction, and in cell and animal studies it has been shown to activate signaling pathways, including one involving the transcriptional co-activator PGC-1α, that promote the creation of new mitochondria.
Animal studies, including work in mice given PQQ-deficient diets, have shown effects on mitochondrial content and function, and some rodent studies report improved cognitive performance measures with PQQ supplementation, but these preclinical findings have only limited translation to controlled human research so far.
Human evidence for PQQ is limited to a small number of small trials, mostly examining cognitive and sleep-quality outcomes in healthy adults over short durations, with generally modest and not consistently replicated findings — this is a considerably thinner human evidence base than compounds like urolithin A within the same "supports mitochondrial function" marketing category, and no trial has demonstrated a clear, reproducible human health benefit at commercially sold doses.