It's refering to floating point numbers or machine precision numbers. (4/10) is considered an exact value. It has infinite precision. (0.4) on the other hand is an inexact value which is subject to the limitations of machine precision numbers.
Symbolic algorithms like Solve are meant to be used on exact values like (4/10). If you use inexact values, some valid symbolic manipulations can be problematic. You can learn more about this in a course on numerical analysis.
