Talent give buff for the digimon in your team. Talent points can be earned from different way, you can obtain in the chest stars on each Battle Stage, and you get more in nightmare stage. By the way, I don't recommend to reset it (500 diamond) that's just a waste and not worth at all. You'll have enough Talent point to complete everything so don't worry if you have made a mistake.
So there is 3 different trees, the first one is about the Formation and will give HP; DMG Immune for the front
and Attack; DMG Rate Bonus for the backrow.
The second one is about the type of the digimon and this tress is divided in 3 other trees, You can up your attack type digimon, your defense type or even your skill one and can affect the triangle weakness. It's recommended to specialize in the branch that your team is focused on. [Attack] > [Skill] > [Defense] > [Attack] > etc.
The Last tree does the same except that instead of affect the triangle weakness that will give special effect like for example for the Skill type Digimon their ultimate skill will have DMG increase while attack types will gain lifesteal. It's recommended to specialize what your team is focused on.