Paste the posting and add your resume. Each term is read with the weight the wording gives it, required, preferred or neutral, and sorted into proven (shown in an experience bullet, a role title or project text), claimed (listed, not shown) or missing, grouped by what kind of skill it is. The check reads what a term means, not how often it appears: Postgres and PostgreSQL are one term, and repeating a word in your resume changes nothing. Nothing is added to your resume for you.
A sentence that says required, must have, minimum or names years of experience marks its terms as required. Nice to have, preferred and bonus mark them as preferred. Terms in neutral sentences sit in between, and every chip names which of the three it got.
Proven: the term, or a known alias such as Golang for Go or Postgres for PostgreSQL, appears in an experience bullet, a role title or a project description, so the work behind it is on the page. Claimed: it appears only in your skills list or summary, so a reader has the word but not the proof. Missing: it appears nowhere on the resume.
No. A term is counted once however often it appears, and a term in your skills list stays claimed until a bullet shows the work behind it. The mention counts you see are for the posting, so you know what it keeps asking for; they are not a target to hit.
Only where your work supports it. A keyword without a bullet behind it does not survive an interview, so the tool never adds terms for you. Copy the missing list as text and check each term against what you have done.
No resume or posting text leaves your browser. The server receives a count that a check ran, with no content attached, so the live counts on the landing page stay honest.