The testability can be defined as follows:
"The degree of ease with which a software system exposes its faults through execution-based testing"
A software system with a high level of testability provides a high degree of exposure of its faults through testing, thereby giving the developers higher accessibility to the system's issues, and allowing them to find and fix bugs faster. A less testable system, on the other hand, would make it difficult for the developers to figure out issues with it, and can often lead to unexpected failures in production.
The testability is, thus, an important aspect in ensuring the quality, stability, and predictability of the software system in production.