What does adding -an to the end of a word do?

Try this first, please. Using the dictionary on this site type in “rootan” then click on Affix Analysis. This will list several possible changes by appending “an” to the word “root” which here represents a Tagalog root word.

@repolyo Perfect. Salamat!

It appears that the Affix Analysis output does not include the case of the suffix '-an' forming an adjective meaning "virtually covered with" e.g. duguan or "having a body-part of unusually large size" e.g., pangahan.
I guess the meaning of "biglaan" could mean "virtually covered with suddeness". (Biglaan also serves as an adverb)

The “an” suffix is not only used to form object-focus verbs but also to form other content words. @repolyo already mentioned adjectives and adverbs. It is used to form nouns too. Hence, the meaning of the resulting word is not as predictable as those verbs formed with other affixes. Moreover, the same word may have another meaning depending on context or on how it is pronounced.Examples: upuán (verb, infinitive/imperative) – to sit on something
upuan (noun) – chair, seatlakaran (verb, infinitive/imperative) – to walk on something
lákaran/lakarán (verb) – to walk (used to refer to people/animals walking about simultaneously or randomly in groups or as individuals, i.e., a group act)
lákaran/lakarán (noun) – walkwaylutuan (verb) – to cook (a group action)
lutuán (noun) – cookware (container), a cooking appliance, a kitchen - meaning depends on the contexttakbuhan (verb) – to run away from danger (a group action), a race (sports)
takbuhan (noun) – a person/persons/a place where people go for help (usually financial or emotional) or shelter
takbuhán (verb, infinitive/imperative) – to run away from responsibility (usually, evasion of debt payment)mabilisán (verb) – to evaluate something as going/moving fast or too fast.
mábilisan (adverb) – hurriedly (mabilisan na/mabilisang ginawa = done hurriedly)
mábilisan (adjective) – hasty (mabilisan na/mabilisang desisyon = a hasty decision)