A purely competitive market means that no seller of a good has a large share of the market, and that customers have a lot of information about that good. Because there are many sellers all selling the same thing, no one seller can sell that thing for a high price. That means that consumers usually benefit because the price is driven down to the lowest sustainable point.
A good example would be something like apples. Lots of people sell apples, and no one seller of apples has a large share of the market. Customers understand apples and generally know the value of what an apple costs. That means a seller couldn't possibly sell an apple for $100, because that customer can buy it somewhere else for cheaper, and they know that it couldn't possibly cost that much. Does that make sense?