The phrase "significant underestimate" means to wrongly guess or judge something to be smaller or less important than it actually is, and the error in estimation is quite large. Full definition
The report suggests that although international public funding for adaptation has increased in recent years, the previous assessments of the costs of adaptation have involved significant underestimates. (uncclearn.org)
There is the potential for significant underestimates in the data in the early portion of the record. (washingtonpost.com)
It's probably a significant underestimate, since most aneuploidies abort early in pregnancy. (healthland.time.com)