"The tests are sticky": There are enough tests so that during a true refactoring the tests will pass, but otherwise the tests will fail (i.e. the attempted refactoring erroneously changed behavior).

The metaphor results from thinking of the tests as an interlocking spider web that catches any bugs trying to fly through it. When there are too few tests, this happens a lot, but as more and more code is written test-first, this happens less often until you can really rely on them.

Velocity often increases at this point since refactorings can be done (and undone) with confidence.