Metadata Character Limits are the fixed maximum lengths Apple and Google enforce on each listing field — the limit is enforced when you enter text, so over-limit copy won't save; truncation happens only on display.
Each metadata field has a hard cap, and the stores differ. On the App Store the headline limits are 30 characters for the app name, 30 for the subtitle, 100 for the keyword field, 170 for promotional text, and 4,000 for the description. See every App Store and Google Play character limit in one table. On Google Play it's 30 for the title, 80 for the short description, and 4,000 for the full description. Going over doesn't cut your text off: App Store Connect refuses to save an over-limit field (its API answers an over-long keyword field with "cannot be longer than 100 characters"), and Play Console lists each limit as a hard maximum. Truncation is a display effect. A name or subtitle that fits the limit can still be cut off with an ellipsis in search results or on a narrow screen, which can hide a phrase you were counting on to convert.
The optimization discipline is to treat every character as scarce inventory — especially in the 30-character title and subtitle and the 100-character keyword field, where the difference between a wasted word and a ranked keyword is a few characters. Counting as you write, rather than after, prevents rejected saves and rushed rewrites at submission.
Example
A subtitle drafted as "Sleep, Meditation & Relaxation Sounds" is 37 characters, and App Store Connect won't save it until it's down to 30 — a counter catches that before submission. Even a subtitle that fits can be cut off with an ellipsis on a small screen, so put the words that matter first. Count as you write rather than after, especially in the 30-character title and subtitle where every character is contested.