In practice
No spaces after commas, no plurals, no repeating words already in the title — every wasted character is a term you could have covered.
Google Play has no equivalent field; there, coverage comes from the description and the title, which is why the two stores need separate keyword sets.
Why it matters
It is the highest-density optimisation in ASO: a hundred characters that decide which searches your app can appear in at all.
Related terms