Metadata Character Limits are the fixed maximum lengths Apple and Google enforce on each listing field — exceed them and your text is silently truncated at submission.
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. 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 throw an error so much as cut your text off mid-word, which can break a phrase you were counting on to rank or 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 the truncation surprises that ship to production.
Example
A subtitle drafted as "Sleep, Meditation & Relaxation" is 31 characters and gets cut to "Sleep, Meditation & Relaxatio" — a counter catches it before submission. Count as you write rather than after, especially in the 30-character title and subtitle where every character is contested.