Subtitle is the App Store's 30-character line beneath the app name — the second-strongest ranking field and a key conversion hook on the product page.
The subtitle does double duty. For ranking, its keywords are weighted just below the title, so it's where your second tier of high-value terms goes. For conversion, it's the first sentence a shopper reads after the name, so it has to communicate the value proposition fast. The best subtitles do both at once: they read as a natural benefit statement while quietly carrying two or three rankable keywords.
At 30 characters, the subtitle forces hard choices. Avoid repeating words already in the title (that wastes ranking weight), and resist cramming in disconnected keywords that read as nonsense — a subtitle that ranks but reads badly costs you the install you just earned the impression for. Google Play has no direct subtitle equivalent; its closest analog is the short description.
Example
Title "Lumen" + subtitle "Sleep, Meditate & Relax Daily" packs three searched keywords into a line that still reads as a clear promise to the user. If a keyword reads as filler to a human, move it to the keyword field and keep the subtitle persuasive.