Type effectiveness is the system where a move's damage is multiplied by 2 against a type it is super effective against, halved against a type that resists it, and reduced to zero against an immune type. Dual type defenders multiply both matchups together, producing quarter or quadruple damage.
Every attacking type has fixed relationships with every defending type, and the chart has had 18 types since Generation VI added Fairy in Pokémon X and Y. Fire beats Grass, Grass beats Water, Water beats Fire is the starter triangle, but the full chart is where battles are actually decided. An Ice move into a Dragon and Ground Pokémon such as Garchomp multiplies two weaknesses into four times damage. A Ground move into a Flying type or a Levitate user does nothing at all.
The chart has changed twice in ways worth remembering. Generation II added Dark and Steel and fixed several Generation I quirks, including Ghost becoming super effective against Psychic. Generation VI added Fairy, which is immune to Dragon and super effective against Dragon, Fighting, and Dark, and the same generation removed Steel's resistances to Ghost and Dark, so those moves became neutral against Steel.
Good team building is mostly chart management. You want offensive coverage across common types and defensive answers that do not stack the same weakness three times. Abilities, moves such as Freeze Dry, and Terastallization can bend the chart, but they bend a chart you still need to know cold.
In play"Electric into a Water and Flying type like Gyarados is four times effective, so protect it."