What does that mean in terms of real-life evolution? How does it apply?
It means the outcome of any evolutionary process cannot be predicted with certainty. For example, we cannot predict that a million years from now, the descendants of humans will be smarter, or taller, or longer lived.
As another example, we cannot look at a proto-planet that happens to lie in the "Goldilocks Zone" of a star and predict that it will someday harbor intelligent life. We also cannot predict that it won't. It is, in principle, possible to assign a probability that intelligent life will evolve on such a planet, but we do not yet have the knowledge to make a reasonable statement about that probability.
One thing it most certainly does not mean is that we cannot make any predictions at all. That sentence has a double negative, so let's walk through it slowly to make sure it's understood. We can make predictions about random processes. While we cannot know the exact outcome of a random process (otherwise it would be a deterministic process), we can know the range of possible outcomes. When we roll two standard dice, we know that the sum might be 2, but it is more likely to be 7, and there is absolutely no chance that it will be 13.
Applying that to evolution, when two polar bears mate, we know that the offspring will be a polar bear and not an ostrich. However, there is a chance that it will be a polar bear with a genetic mutation and if that mutation is favorable, there will be more chance that it will be passed on to future generations. If the population of bears with such a mutation is isolated, there might be more mutations in the future, and at some point, the variations in the population of bears will be sufficiently large that the descendants of the mutant bears won't be able to mate with the original polar bears. If that's the case, then the descendants of those original polar bears will not be polar bears.
Finally, back to the original example about intelligent life on a protoplanet, we cannot say with certainty that intelligent life will evolve, or will not evolve. If it does evolve, we can say it did so "by chance".