One gets this message when one has decimal points in the numbers in the equations. Inexact floating-point arithmetic is normally used to compute with these numbers. Solve[] converts the numbers to rational numbers represented exactly by a ratio of integers. It then solves the exact system and converts the solution back to floating-point numbers. Solve[] generally expects exact input, although it usually deals effectively with inexact input. But because of this expectation and because it rounded the numbers to rational and then rounded the solutions back to floating-point, it issues a warning message. If the answer seems wrong, the user knows a possible, albeit unlikely, reason for the discrepancy is the numerical conversions. If the answer seems right, you can ignore the message, and this is the more common case in my experience.
