Just read up on initialism. That has helped.
Yes, I think I get this now after looking online as well.There are many forms of abbreviations, which – as the Latin derivation of the word shows – means that they are shortened forms of a word or words. Most abbreviations are of single words, so they usually need to be longer than the first letter of the word fin the interest of perspicuity: e.g. 'Prof.' for 'professor or 'Dr' for 'doctor'. However, there are single-letter abbreviations of single words: the single letter p can mean 'page', since the context where it is found makes it obvious what it refers to. Similarly, the French abbreviation for 'Monsieur' is the single capital.
An acronym – as the Greek derivation of the word shows – is a shortened form of a group of words using their initial letters. Strictly speaking there is no requirement for this to be a pronounceable word, but many do restrict the word to this kind of abbreviation, because it allows for an easy way of distinguishing shortenings like 'FBI' or 'TMI', which no one would, or even could, pronounce as single words, and 'NATO' where nobody says 'Enn Ay Tee Oh'.