A p value of 0.01 means that there is a 1% chance that the difference between treatments is due to random chance rather than to a real difference between the treatments (the two colors of mice on either soil). This 1% value is usually taken to mean that the difference between treatments is quite significant, the statistical test confirms confidence in the result.
Work Step by Step
Statistical tests are designed to produce a probability that the experimental differences between treatments are not just due to random variation/chance. It is common to use 1%, 5%, or 10% as criteria for deciding that results are confirmed/differences are real. Just be careful to realize that these probability levels are just predictions, not magic lines in the sand. 4.9% isn't much different from 5%, even though some scientists choose not to acknowledge it.