It doesn't say that hodiaŭ can be a preposition or an adjective, it says that the -aŭ words can. The text I posted is copied from the Wiktionary page I linked to.
It doesn't have a meaning per se.
There are a number of particles (mostly adverbs) ending in -aŭ (hodiaŭ, morgaŭ, hieraŭ, preskaŭ, almenaŭ, kvazaŭ, antaû, kontraû - perhaps a few others that I'm forgetting) but those aren't related to words ending in another suffix instead of -aŭ; instead, those letters are simply part of the word stem. (This also means that you can't create new words by adding -aŭ to anything else.)