It is just the fancy word for a leap year.
From Wikipedia: From Latin bisextilis annus (“bissextile year”), from bisextus + –ilis, from bis- (“two; twice; doubled”) + sextus (“sixth”) + dies (“day”), from the Julian calendar’s original reckoning of its quadrennial intercalary day as a 48-hour February 24, subsequently distinguished as the two separate days of the sixth day before the March calends (sexto Kalendas Martii) and the “doubled sixth day”. February 24 is now normally understood as five days before the first of March, but was called the sixth by the Romans owing to their inclusive counting of dates.
There are not many numismatic items associated with leap year. Shown are two of the three that I have encountered. The first is a one Scottish pound note from the British Linen Bank dated February 29, 1968. The other is a $25.00 US War Savings Bond with an issuing agent’s date stamp of February 29, 1944.
The only other numismatic item I know of with this date are the unissued Bangko Sentral ng Pilipinas notes printed during the Japanese occupation which have an authorization date of February 29, 1944.