Say that it's cloudy today. "It's going to rain", a friend says. If it does rain, there's a good chance that she'll go on to say "Look, I knew it was going to rain, it was cloudy." This is hindsight bias at play. Clouds don't always mean rain, and this friend is in danger of incorrectly assuming that she can predict the rain.
Hindsight bias can affect us when we're interviewing for jobs, or betting on a sports team or watching an election. To avoid bias, it's better to observe the actual probability from many instances. When that's impossible, perhaps best to remember that the event was actually unpredictable.