“Thereon” just means “on it” “on this / that” or “on them”. It was a common pattern of combining a preposition with a 3rd person (inanimate) pronoun in Old English. There are many examples, such as “thereafter”, “therein”, etc. Nowadays this construction sounds rather old-fashioned.

This is an excellent, excellent question. Clearly, "thereon" is modifying "reports": It is a meeting to discuss the annual accounts, and the auditor's reports *about those accounts*. So, here we have a word that dictionaries (including the OED) describe as an adverb, which is modifying a *noun*. At first, I thought that this might be because the noun "report" is the nominal form of the verb "to report", but nope--"thereon" works with completely standard nouns as well: "she looked at the table and the books thereon"="she looked at the table, and at the books *on that table*." Then, I thought of other places where adverbs can modify pronouns: "almost everyone" and "hardly anyone". Here, though, the adverb isn't actually modifying the complete pronoun: it's modifying the determiner "every" or "any"--grammatically, "everyone" and "anyone" act like two-word phrases, and we just write them as single words because of historical precedent. And a similar thing is happening here. "Thereon" clearly isn't a traditional adverb. And even though it is modifying a noun in this sentence, it isn't an adjective, since it comes after the noun instead of before it. Grammatically, it is acting like a one-word prepositional phrase: prepositional phrases like "on the mountain" come after the thing they modify, and can either modify a verb (he danced on the mountain) or a noun (the trees on the mountain were all very short.) "Thereon" works exactly the same way: "I wrote my report, and he commented thereon (=on my report)"; "I looked at the report and his comments thereon (=on it)." Most "there-" adverbs (like therefrom and therein) act similarly, as does the word throughout ("I looked at the book, and the highlighting throughout"). As far as I know, there's not a common grammatical category that specifically names one-word prepositional phrases. In general, when you encounter a word that doesn't fit any other category, it is common to just call it an adverb. In the words of Wikipe

Thereon basically means next or after. So in you’re sentence, “a meeting to discuss the annual accounts and the auditor’s report thereon”, another way to see it is like “first a meeting and then we’ll take a look at the auditor’s reports”

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