The property demonstrated in the equation you provided is called the associative property of addition. According to this property, it doesn't matter how you group the numbers when adding them; the sum will remain the same.
In the equation −1.4+(−2.6+3.8)=(−1.4+(−2.6))+3.8, you are associating the numbers -2.6 and 3.8 first, and then adding the result to -1.4. Alternatively, you could associate -1.4 and -2.6 first and then add the result to 3.8. Both expressions will yield the same sum.
So, the associative property of addition states that you can regroup the numbers being added without changing the final sum.