Reading the chart
Every move has a type, and every Pokémon has one or two. The chart says what a move is worth against a type: ×1.6 when it's super effective, ×0.625 when the defender resists, ×0.39 where the main series games give an immunity. Everywhere else, ×1.
Against a dual-type Pokémon, multiply both cells. An Ice move against Dragonite (Dragon and Flying) is worth 1.6 × 1.6 = ×2.56: the best possible pick. The other way round, a Fire move against a Water and Rock Pokémon is worth 0.625 × 0.625 = ×0.39.
What adds to the chart
Same type: a Pokémon using a move of its own type gets ×1.2.
Weather: weather that boosts the move's type adds ×1.2.
Friendship in raids: up to ×1.1 with a best friend in the same raid.
Why it differs from the main series
Pokémon GO keeps the same strengths and weaknesses as the series, with its own numbers: ×1.6 instead of ×2, and no full immunity. A Ghost Pokémon still takes a little damage from a Normal move.
In raids
For a raid boss, look first for a ×2.56 move if the boss has two shared weaknesses, otherwise a ×1.6 one. Avoid moves the boss resists: at ×0.39, even a strong Pokémon barely helps.
Frequently asked questions
What does “super effective” mean in Pokémon GO?
A super effective attack deals 1.6 times the damage. Against a dual-type Pokémon the multipliers stack: up to 2.56 times when both types are weak to it.
Are there immunities in Pokémon GO?
No. Where the main series games cancel the attack, Pokémon GO cuts it down: it only deals 0.39 times its damage. Against a dual-type Pokémon it can drop even lower.
Does the same-type bonus count?
Yes, on top: a Pokémon using a move of its own type gets 1.2 times, on top of the chart. Weather that boosts the type adds another 1.2 times.