An acronym is a word that when it’s abbreviated you say aloud like it’s a proper word. Take for instance Self Contained Underwater Breathing Aparatus… When you say it aloud it creates a word. Scuba.
Initnialisms are abbreviations that when you talk about them don’t get turned into words but you spell them out by letter. Like FBI. You don’t say something like “Ffffbee” cus it’s kind of awkward. You refer to it by it’s initials hence initialism.
Sometimes you get a weird distinction where something could be easily both but culture makes it one or the other like the World Health Oganization could have easily been an acronym but for clarity sake they chose to propagate it as the initialism. W. H. O.