Shift-left testing is an approach in Software development that proposes that tests should be done earlier in the development process rather than allocating test and QA time only at the end of the project timeline.
The idea is to use tests not only for validation, but to drive implementation and feature discovery, much like Test driven development. When we test as we develop we end up finding not only bugs but also architectural flaws much earlier, giving us a better chance to steer the project and implementation in the right direction.