One simplified answer is that it helps reduce ambiguity in a language full of homophones. There's more to it than that, of course. An interesting read about it is here: https://japanese.stackexchange.com/questions/51756/if-kanji-are-necessary-to-disambiguate-homophones-how-come-its-still-used-bei
There are actually not that many true homophones as it might seem, but there are a lot of words written the same way. The thing is that Japanese has what you call accent tone. Words can be written the same way, but differ in intonation, and thus be identifiable as different words in spoken Japanese. In writing, however, this is not shown directly, and by showing the kanji you would know the meaning of the word, and then the pronunciation. This is also why I try to learn the basic meanings of the kanji and not every single reading. Not sure how that would work out, but it's worth a shot.
-mono makes it a noun, more specifically a "thing". [nomimono => drank thing => beverage] [tabemono => eaten thing=> food] [kimono = is the thing being worn on shoulders => special jacket] [bakemono = shapeshifting/hidden thing => shapeshifting monsters]
To return to the verb form you replace -mono with -ru. [taberu = to eat] [nomu = to drink (exception because already ends with -u)] [kiru = to wear (on shoulders)] [bakeru = to transform or be disguised (think tanuki or bakeneko)]
There are other ways to conjugate them, such as -imasu, which should be pretty hammered in with the previous lessons by now.
くだ is like the old equivalent of きの. き is this instance refering to 木 (tree). の being the possesive marker between nouns. 木 apparently could be read くin the past and だ was used as a possesive marker. So it literally means "thing of a tree". However, like you will see later on, some words like that with an old reading of a kanji have changed the kanji to fit more with the meaning of the word. So くだもの doesn't use the kanji 木 anymore but the kanji 果 instead, one of its meanings being fruits. That's also why the reading of 果物 might look like a strange reading if you know the readings of 果 and 物 since 果 usually isn't read くだ.