Here is a rather long but interesting quotation which is particularly relevant for this course. The quotation comes from the "Unua Libro" by Zamenhof, the initiator of Esperanto. It is followed by an explanatory comment by Claude Piron, an outstanding Esperanto author. However, you might find this page more interesting to read when you have already done some lessons in the course, because you will then probably better understand the essence of the texts below.
"I introduced a complete dismemberment of ideas into independent words, so that the whole language consists, not of words in different states of grammatical inflexion, but of unchangeable words. If the reader will turn to one of the pages of this book written in my language, he will perceive that each word always retains its original unalterable form—namely, that under which it appears in the vocabulary. The various grammatical inflexions, the reciprocal relations of the members of a sentence, are expressed by the junction of immutable syllables. But the structure of such a synthetic language being altogether strange to the chief European nations, and consequently difficult for them to become accustomed to, I have adapted this principle of dismemberment to the spirit of the European languages, in such a manner that anyone learning my tongue from grammar alone, without having previously read this introduction—which is quite unnecessary for the learner—will never perceive that the structure of the language differs in any respect from that of his mother-tongue. So, for example, the derivation of frat'in'o, which is in reality a compound of frat "child of the same parents as one's self", in "female", o "an entity", "that which exists", i.e., "that which exists as a female child of the same parents as one's self" = "a sister"—is explained by the grammar thus: the root for "brother" is frat, the termination of substantives in the nominative case is o, hence frat'o is the equivalent of "brother"; the feminine gender is formed by the suffix in, hence frat'in'o = "sister". (The little strokes, between certain letters, are added in accordance with a rule of the grammar, which requires their insertion between each component part of every complete word). Thus the learner experiences no difficulty, and never even imagines that what he calls terminations, suffixes, etc.,—are complete and independent words, which always keep their own proper significations, whether placed at the beginning or end of a word, in the middle, or alone, and that each word may equally well be used as a root word or as a grammatical component." (L.L. Zamenhof, 1887)
You will probably be surprised that, for Zamenhof, language elements such as o and in are words. In my opinion, he used those terms to emphasise that the language consists of unchangable units that can be added to each other without ever bringing about a change of form within any of those units (changes of this sort occur frequently in western languages such as English: "foot > feet"; "come > came"). My impression is that he saw a description of the Chinese language somewhere, with examples - perhaps he even took a Chinese grammar in his hands - and he noticed that the structure of that language has many advantages, on the one hand because of its perfect regularity, and on the other hand because of the ease with which it is possible to express complex concepts by combining simple words. Now, in the nineteenth century, texts about Chinese, which were based on writing without consideration of the spoken language, generally used the term 'word' when in fact the text referred to the basic immutable units that the language consists of. Probably for this reason, Zamenhof applied the same terminology which, confusing though it might be, is not really inaccurate, if we consider for example that the word in is not essentially different from words such as patr or frat: it too can be used autonomously with any grammatical ending, making it entirely different from the suffices of the inflexional and agglutinative languages.
In the preface to Vere aŭ Fantazie I gave the name "subwords" to the components that together constitute a full word. Using that terminology, you could say that fratino is a word that consists of three subwords: frat, in and o.