Let's refresh what a Trigger and an Action are:
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.
An Action is something that is done. Examples of actions are: "turn on the light", "lock the door", "send an email", etc.
Simply put, Triggers and Actions are what tell flows what to do and when to do it.
To select and configure a Trigger or an Action, simply tap on the thing you wish to configure, select the appropriate trigger, or action, chose the settings appropriate for your flow scenario, and tap Save.
Image: Configuring a Trigger in Location
Tip: Flows always begin with a Trigger and end with an Action. There may be multiple Triggers and Actions within a flow, but something (a Trigger) has to initiate a flow, and the flow has to have a result (Action).