What this schema validator checks
It finds every JSON-LD block on the page and checks three things. First, that the JSON itself is valid: one stray comma makes Google ignore the whole block, so the tool shows the exact line and column. Second, that each item has the properties Google requires and recommends for its type. Third, that values are in the right format: ISO 8601 dates, plain-number prices, three-letter currency codes and full https:// addresses.
It follows @graph and @id references, so it understands markup from SEO plugins like Yoast and Rank Math that link items together.
Errors versus warnings
Errors (red) stop an item from earning a rich result: a missing required property or a value Google can't read. Warnings (amber) are recommended properties. They're optional, but filling them in gives Google more to show.
Where to put schema markup
Put the <script type="application/ld+json"> block in the page's <head> or anywhere in the <body>. Google reads both. Only mark up what's visible on the page: invented ratings or prices can lead to a manual action.