Potassium-40 is a fairly unstable isotope, although the half life is nearly a billion years.
Because bananas have so much of this isotope, there is enough decay to generate one positron (approximately) every 75 minutes.
A positron is basically the opposite of an electron. It has the same mass as an electron, but a positive charge instead of a negative one. It is the electron’s antimatter doppelgänger.