Forest is chiefly a productivity app. You set some length of time, and the app starts a timer and plants a little virtual tree. As long as you don't do anything else with your phone for that length of time, the tree grows, and over time you can accumulate a nice little grove. If you interrupt the app to do something else with your phone, the tree dies and you have a dead tree in your forest.
The app is Pomodoro-friendly; it's balanced to reward you (in virtual coins) a bit more for setting focus periods of 25 minutes or longer. However, it also provides coins at a higher rate, and grows more elaborate trees, when longer times are set, up to 2 hours. So if you want to be strict about Pomodoro timing, and just plan for 2 hours' worth of it at one stretch, you may need a separate timer for the 25-minute chunks. The coins can be used to buy additional designs of trees; I like to use specific tree designs for the specific kinds of work I was doing while the app was running.
What turns this into a "green" app is the fact that if you pay for the pro version--a one-time expense, not a subscription--then the coins you earn by growing the virtual trees can be put towards planting real ones via Trees for the Future. At 2500 coins, a real tree costs somewhat more than the most expensive virtual trees (2000 maximum, so far).
If you're glad I told you about Forest and you'd like for both of us to get 500 coins, go into Settings -> Invite Friends and Get Rewards -> the % in the upper right and enter this number: 4QCYGM6SG