There are 7 different actions that players may choose from on their turn. Players may do 1 or 2 actions on their turn, in any combination (for example: 2 of the same action).
A) Play a card from your hand.
B) Use a standard project.
C) Claim a milestone.
D) Fund an award.
E) Use the action on a blue card.
F) Convert 8 plants into a greenery tile (which gives an oxygen increase) as described on the player board.
G) Convert 8 heat into a temperature increase as described on the player board.
By choosing 2 actions, a player can surprise the others, by doing more than they expected and beat them to some goal or bonus.
Choosing only 1 action can be useful too - it can be used for waiting out your opponents or just to do a no-brainer while thinking about your more important decisions.
You can choose to take 1 or 2 actions on your turn. If you take no action at all (pass), you are out of the round and may not take any more actions this generation, but as long as you take actions, you will get more turns. When everyone has passed, the Action phase ends.