It is a glycoprotein which is extracted from pineapple (Ananas comosus, Ananas sativos) and other fruits with a protease-like property (see specific info) and activity as proteolytic-like enzyme with the ability of protein digestion.
It is used in the food industry to softening the meat. 

 In medicine no proven utility has been shown. Effectiveness as an anti-inflammatory and digestive-enhancer is dubious. It has been topically used on local inflammations and burns. 

It has been proposed for oral treatment of breast plethora (Murata 1965), yet there has been an only isolated and very old study which does not constitute an enough evidence to widely recommend it (Snowden 2007, ABM 2016).
The few studies on oral protease utilization to alleviate breast plethora found out discordant results wiht low levels of evidence (ABM 2016, Mangesi 2016). 

At latest update no published data were found on its excretion into breastmilk.
Because of a high molecular weight and a non-significant plasma level concentration (5 micrograms / L) the passage into breastmilk is seemingly unlikely.