High firm concentrations in a given market may not translate to market power. Even in markets where only one or a few firms can efficiently operate (for example due to economies of scale), it is possible for competition to work.
A market is said to be contestable when barriers to entry and exit are so low that the threat of potential entry prevents the incumbent from exercising market power.
In perfectly contestable markets there are no barriers to entry or exit. With free entry into and exit from the market, the threat of potential entry will constrain the behaviour of incumbent firms. Should an incumbent firm increase prices above the normal level of profits, then new firms will enter the market and force prices down again.
Contestability requires that there are no sunk costs for market entry. That is, should an entrant fail, it can recover its fixed costs (for example by selling assets or reusing them elsewhere).