A skill is a type of ability, such as magic, or a special move. There are usually many types of skills, such as healing skills, damaging skills, and skills solely used for traversing the world more quickly. Usually, damaging skills have different elements, such as fire, ice, and thunder. In addition, usually there are multiple skills of the same element which have differing effectiveness when used. Usually, skills cost a certain amount of "magic points" to cast, and if the user does not have enough magic points, the skill cannot be cast.
Not all games adhere to this description, as some games have different mechanics for skill usage. Some games use a second stat in addition to MP that skills may require to use, either in combination to MP or instead of MP, which gradually recovers during the battle.
In RPG Maker, skills can be created and configured to be used in combat or outside of combat, or both. Skills are created and modified using the database. Many aspects of skills can be modified, such as MP/TP cost, elements, the amount of damage or healing it does, as well as any effects it can cause, such as adding states to the character.