Seed packets are found Seedy levels. They help contribute towards your Plants family Skills. Seed Packets may be purchased with Gems.
Master Skills replace the leveling system from PvZ2. Master Skills are unlocked after beating Zomboss for the first time. Master Skills are unique to each plant class and boost a certain aspect of those plants. An example would be the Nut classes' 1st Master Skill which is Explod-O-Nut which allows them to explode when fully eaten. Master Skills require Skills which are obtained by committing a few seed packets and sometimes coins to a skill for a plant family. An example of a regular skill would be the Pea classes' Heavy Peas upgrade which makes their peas push zombies back every 4th pea they shoot.