What happens if my structured data is invalid?
It gets ignored, and you are not told. Structured data is all-or-nothing per block: markup that does not parse is discarded rather than partially understood, so the effort of adding it produces exactly the same result as never having bothered.
What it actually is
Validity here means two separate things. The block has to be well-formed, so a machine can read it at all — a stray comma is enough to fail that. And it has to declare which vocabulary it is speaking and what kind of thing it describes, because without those the contents are just an anonymous bag of values with no agreed meaning.
Why it matters
Invalid markup is worse than absent markup, because absent markup is at least honest about itself. Broken markup shows up in your source, looks like the job was done, and is silently thrown away by every machine that reads it. Teams routinely believe this box is ticked for years. There is no warning, no error, and no visible difference between markup that works and markup that does not.
What good looks like
Every block parses cleanly on its own, declares the vocabulary it uses and the type of thing it describes, and carries the properties that type is expected to have rather than a subset. Where a page contains several blocks, they agree with each other rather than describing the same thing two different ways.
Being honest about it
Validity is a floor, not an achievement — valid markup that says something useless is still useless, and this check deliberately does not judge whether what you declared was worth declaring. We flag it separately from presence because the failures are so different: one is a decision not to do the work, the other is the work having been done and quietly wasted.
Where most sites go wrong
The most common failure is a template that builds markup by pasting values into a string, so any apostrophe or quotation mark in a product name breaks the whole block. It works perfectly until someone adds an item with a possessive in the title. The second is a plugin conflict: two systems each add their own description of the same page, both technically valid, contradicting each other about the same facts.