Unicode has four main normalization forms. Their codes are NFC, NFD, NFKC, NFKD. Wikipedia has a good explanation of the topic.
The normalize() method returns the string normalized according to the form you specify, which you pass as parameter (NFC being the default if the parameter is not set).
I will reuse the MDN example because I’m sure there is a valid usage but I can’t find another example:
'\u1E9B\u0323'.normalize() //ẛ̣
'\u1E9B\u0323'.normalize('NFD') //ẛ̣
'\u1E9B\u0323'.normalize('NFKD') //ṩ
'\u1E9B\u0323'.normalize('NFKC') //ṩ