Shift-left accessibility testing borrows the shift-left concept from general software quality practice: move testing earlier, into wireframes, design mockups, and in-progress code, rather than treating it as a final gate immediately before release, when issues are far more expensive and disruptive to fix.
In practice this looks like checking color contrast during design review, running automated accessibility linting as code is written, and including accessibility criteria directly in a feature’s definition of done, so problems get caught and corrected while they are still cheap to change.