That's just one example of it.

Other examples:

* Replacing automated tests with (quicker) type checking and running it on a git commit hook instead of CI.

* Replacing slower tests with faster tests.

* Running tests before merging a PR instead of after.

* Replacing a suite of manual tests with automation tests.

etc.

More broadly it includes moving activities that are normally performed at a later stage so that they are earlier in the process. The idea is that defects found later in the process are more costly.

I would say tests first/TDD is a form of shifting left, but it can encompass more than that.

It can also mean manual testing earlier. Valve credits playtesting early and playtesting often for their success with Half-Life, Half-Life 2, and other games.

No, not really. It's a concept from the bygone era, before X-as-a-service took over a lot of software categories. It was intended to minimize friction between the QA and the development teams at the time the product was handed to QA. If the product had multiple defects, incrementally discovered, it would have to travel back and forth between the QA and the developers, stalling the development of the next version and generally creating a lot of noise and inconvenience due to the surrounding bureaucracy.

There's no need to write tests upfront for you to shift left. All shift left means is that testing happens during development. Whether you start by writing tests and then write the actual program or the other way around -- doesn't matter.