Properties of a box are: being angular, empty inside, and we put/store stuff in it. Now comes the abstraction: we take this properties and see what other things they characterize. Let's say another item has these, but also has additional properties, e.g. it can roll around, it's for transportation, it makes noises when used, etc. It's clearly a different thing than a box but also bears the properties of a box. In English they call it a "car". But how about we make a word that reflects the similarity to a box, but also implies that this thing bears additional functions. So call it a boxcar.
But we want usable words and not compound words, especially because in time these words can grow ginormous. We don't want to stick whole words to each other, only letters or syllables.
Root words are the most basic words covering the most basic concepts which all the other concepts - and with them words - are growing from. Root words are also as simple as a syllable or even a letter.
So let's call our "angular, empty thing we put/store things in" a bo. And let's say the word bo contains all these abstract properties.