The Test Match Between Australia and England

The Ashes test match between Australia and England takes place at the Adelaide Oval. The ultimate form of cricket is considered this five day game, which is called a test match. When you say test match though, that makes it sound like it's not the actual matches for the ashes. No, that's not what it means. What is like the test match just threw me. That sounds like it's a testing in my head reads as like the semi-finals for the ashes... I'm just a bit confused by what that word means.