Audio erotica is explicit fiction performed by a narrator. Most of it is written for audio from the start, in scenes of five to twenty minutes, rather than being a book someone later read aloud.
Not the same as an erotic audiobook
An audiobook is a novel in your ears: hours long, one voice, a plot. Audio erotica is built for a single sitting, often second person, often with sound design, and it does not expect you to remember chapter four. Both exist, and the big audiobook retailers carry an erotica category of their own, but the apps built for this are selling the short form.
What the apps actually sell
A catalogue of original recordings and, usually, a subscription. Prices cluster between about five and sixteen dollars a month, with annual plans well below that per month, and the free tiers range from a handful of rotating stories to nothing at all. Several services publish no price on the web and only inside the app store listing, which is worth knowing before you compare them.
The free end
Free audio erotica exists in quantity on general story sites with audio sections and on community recordings, where performers post work for nothing. The quality varies more than in the paid apps and the tagging is better, which is a fair trade for many listeners.
What to check before paying
Whether the narration is original or licensed, whether you can download for offline listening, what the free tier really covers, and what the charge looks like on a statement. Those four answers differ across services far more than the catalogues do.
Why so much of it is second person
Because it works. "You" puts the listener in the scene without describing them, which keeps the story open to anyone playing it, and it suits a form that is usually heard with eyes closed. Not everything is written that way, and some listeners actively prefer overhearing two characters rather than being one, which is worth knowing when a catalogue describes itself as immersive.
See also narrator and the glossary.