The brain is a biological organ that many life forms have. Its primary function is to process data from various sensory organs, such as eyes, noses, tongues, etc. Depending on the data, the brain can make decisions as to what actions to take, and because it's wired, through a nervous system to the rest of the body, can invoke muscular activity. In many ways, it's a lot like a computer.
One way that the brain is not like a typical desktop computer, is that it is a vast neural net. Instead of having a "processor" that does individual mathematical equations, input data is ran through a network of interconnected neurons in a form of parallel processing. The processing happens due to the weights of the relationships between neurons.
Here is the important point to the neural net model of the brain - The brain is effectively a massive pattern recognition engine.
Pattern recognition mostly what our brains do.