What is Test-driven
How much test code a developer writes alongside the code that runs the product.
In plain terms
Tests are code that checks other code, and writing them is a professional discipline that pays off in fewer production failures. A developer's ratio of test code to product code across their repositories is a rough, checkable read on that discipline. Personal projects run lower than employer work here, so the pattern needs to show up across several repositories before it means much.
What it tells you about a developer
A developer who tests their own side projects, where nobody is making them, has made quality a habit. Expect fewer late surprises and an easier review culture around them.
Related
Read the lesson on Test-driven More in How software gets built