What?, Huh?

what / that thing (used in place of words you can't remember at the moment.)

what

what is this?

to say goodbye

understanding quickly; sharp; keen / quick-start guide

easy to understand / (of a person) able to be read like a book

to be evident / to be obvious / to understand completely

分かり切る: to be evident, to be obvious, to understand completely

You'll be convinced if you listen to me.