Magery is the skill to determine how successful a player will be at casting Evocation and Manifestation spells. As of PR 10, v1.0.0 (July 8, 2020) Evocation and Manifestation have been combined into Magery as one skill. Prior to PR 10, they were separate skills.
The minimum recommended skill needed to cast a spell is (difficulty level * 10). You will fail a lot at the minimum; however, you will be able to get skill gains. Anything less will make it hard to gain skill.
An example is Lighting which is a difficulty 4 spell. It's recommended to not start casting this spell until 40 Magery.
To have a very high or 100% chance to cast a spell you need ((difficulty level * 10) + 30.1). To use Lighting again, you need 70.1 magery to be 100% successful with no failure.