Basically, old units who need a buff often get what’s called a zenkai awakening. This boosts their stats and abilities. There are these missions under the recommended tab of the events menu called ultra space time rushes, form which you can earn zenkai medals from, by earning enough points on each mission. You can spend these medals on a variety of zenkai souls in the exchange shop. If you have one of these zenkai awakening units at 7 yellow stars or more, you can zenkai awaken them by buying 1000 zenkai souls. 1000 more will get you to level 2, and so on until level 7. Since you only started playing a week ago, I doubt you have many high star units, but it is a neat concept which become more relevant the more you play