Unawares means being caught by surprise, without noticing something until it happens. When a rainstorm catches you unawares, you suddenly realize it's pouring but you don't have an umbrella. When a teacher calls on you unawares, you weren't paying attention and don't know what question was asked.

The word describes that moment when something happens before you're ready or aware. A cat might creep up on a bird unawares, approaching so quietly the bird doesn't notice until it's too late. A difficult math problem might catch you unawares on a test you thought would be easy.

Notice the word ends in -s: you say someone was caught unawares, not “unaware.” The -s gives it a specific meaning about the moment of surprise itself. If you're simply unaware of something, you just don't know about it. But if something catches you unawares, there's that sudden jolt of realization when you discover what you didn't know.