From Bulbapedia, the community-driven Pokémon encyclopedia.
A retreat cost is the number of Energy attached to a Pokémon that must be discarded in order to retreat to the Bench and bring up a Pokémon from the Bench. Retreat costs printed on a card range from 0 energy (often called a "free retreat") all the way up to 5 energy. They are marked by Colorless () Energy symbols. A player may only retreat once per turn.
Factors influencing printed retreat cost
Typically, Pokémon that have higher speed in the core series tend to have lower retreat costs in the TCG. As a result, Lightning Pokémon tend to have low retreat costs while Metal Pokémon tend to have higher retreat costs.
Although Basic Pokémon often had retreat costs of 0 in early sets, most sets produced since the Expedition Base Set do not contain any Basic Pokémon with free retreat costs. Virtually all modern Pokémon with no retreat costs are evolved, and many are Flying-type Pokémon.
Effects that alter retreat costs
Several Pokémon are capable of altering retreat costs, usually through Pokémon Powers. In most cases, increases in retreat costs are done by fixed amounts, while reductions may be either fixed amounts or decreases to 0. Increases are calculated first, meaning that if an increase to the retreat cost and a reduction of the retreat cost to 0 affect the same Pokémon, the Pokémon's resulting retreat cost is 0.