Adverbs are words that describe situations - how, what, when, where, or why something happened. They always stay the same - you don't change their endings.
In particular, locative adverbs give information about a location. It makes sense from the name, yeah?
Locative adverbs go with the questions Wo? (Where?), Wohin? (Where to?), Woher? (From Where?)
When answering the questions "Wo?" / "Wohin?" / "Woher?" you can use either a preposition and a noun, or an adverb.
What's the difference between using a preposition and an adverb?
With a preposition + noun combo, we are describing a specific, unchangeable location.
„im Keller“ ⇒ nowhere else
With an adverb, we are describing a non-specific, relative place that depends on the point of view of the speaker
„Unten!“ ⇒ It can be anywhere below, not necessarily just in the basement. And if you said the same sentence from a different location, "Unten!" would mean a different location.
Other adverbs answering the question "Wo?": rechts, links, oben, unten, drüben, hier, dort, hinten, vorn, irgendwo, innen, außen, nebenan, nirgends,…
* For info about "hin" / "her", "raus" / "rein", and "rauf"/"runter" look at the explanation in this lesson: The Locative Adverbs Hin and Her.
More information about locative adverbs.
To understand the differences between the version with preposition and adverbs, take a look at the section on locative prepositions.
Here is a more detailed explanation about the locative adverbs "hin" und "her".