Triggers and Actions are essential parts of a Flow. They are the building blocks of creating automation in your life.
A Trigger is what starts a Flow. It is essentially an event. Examples of triggers are: "when the door is opened", or "when the temperature rises above", or "when I am at home", etc.
Image: Triggers in the Nest thermostat
An Action is something that is done. Examples of actions are: "turn on the light", "lock the door", "send an email", etc.
Image: Actions in the Nest thermostat
Different things in Stringify have different triggers and actions. For example, the Weather thing has triggers such as "when the temperature changes", whereas the Nest thermostat has actions such as "set the temperature to...".
By combining things, and selecting desired triggers and actions, you can create powerful flows that automate many aspects of your life.