What is consumer surplus? Why is it important?

Consumer surplus is an important concept in economics. Essentially, it is the extra amount that a consumer is willing to pay for a given good or service. It is the difference between the current amount paid and the maximum the consumer would be willing to pay for it.
Consumer surplus is often dictated by the amount of elasticity of demand there is for the good/service. So the level of which the demand for the product will change dependent on the price. Products with an inelastic demand, for example, are more likely to have a large consumer surplus. They are willing to pay a lot more to consume the product than what they are currently paying for it.
Firms that have 'monopoly' power can exploit the consumer surplus of consumer. This is because as the marketplace for product is not competitive i.e. they aren't multiple firms that the consumer could go to, the firm can set the market price. Therefore, as there are no alternatives, the firm can set the price to a point they can exploit the consumer surplus.