Definition of covered market
A market is covered if all consumers will choose to buy from at least one of the firms at the prevailing prices.

For example, consider a standard Hotelling model with two firms who are located at opposite ends of the unit interval. Suppose that each firm charges that same price $p$ and that consumers are uniformly located on $[0, 1]$. Finally, suppose that every consumer has unit demand and the same utility function $u = v - tx - p$ where $v$ is their valuation for the good, $t$ is the cost of transportation and $x$ is the distance that they travel to the firm from which they buy. In this example, every consumer will choose to buy a good if and only if the consumer located in the centre chooses to buy the good (since she has the furthest distance to travel). This is equivalent to asserting that

$$ v - 0.5t - p \geq 0 \iff v \geq 0.5t + p .$$

That is, the market is covered when the valuations of the consumers are sufficiently high relative to the price and cost of transportation.