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. Daily "Focus Challenge" quests also ask for a random type of tree to be planted each day for bonus rewards, so you may want to strategically buy trees as you get asked for them.

What makes this a "green" app is the fact that if you pay for either of the paid tiers (either a one-time "Pro" version, or a "Plus" 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 a bit more than any of the currently-available virtual ones (2000 max, currently). It's also probably still worth buying some virtual trees to use for daily quests; each needs to be unlocked only once, and the quests enable faster earning thereafter.