This is a tool used to gain more information about your Pokémon. Specifically, how their individual type[s] match up against all other types in the series. This will give you the knowledge of when to send out certain Pokémon in battle and allow you to better structure your Pokémon teams.
Incoming damage multipliers affect how much damage an incoming attack will do to your Pokémon. For example, due to Bulbasaur having Grass as one of its types, it would have a 2x multiplier for incoming Fire attacks. This means it will take double damage from incoming Fire type attacks.
Your team evaluation is calculated by adding the multipliers for each
member of your team to come up with a total incoming damage multiplier
for each type. For example, if six of your team members have a 2x
multiplier for incoming Water type
attacks your team's overall score would be 12 for
Water type. Higher scores mean your team
is more vulnerable to an attack type and lower scores mean that your
team is more resistant to an attack type.