Explanation

Box 1: Trigger
A trigger is something that triggers or starts the workflow. It could be a new file uploaded to a document library, a new item created in the list or an email sent to your Inbox - something that will start (kick-off the workflow).
There are two types of triggers:
Automatic trigger is pre-programmed into the workflow based on a certain rule (i.e. new document or item created, email sent, etc.) Manual Trigger is a trigger initiated by a user. Unlike with automatic trigger above, the workflow will only start when initiated manually by the end user (for example, from the Document Library for a selected file). Manual trigger is only supported for certain types of workflows (i.e. Get feedback from your manager for the selected file) Box 2: action An action is something that occurs as a result of the workflow.
Box 3: expression
Sometimes users just need to do basic operations like getting the current time, adding numbers together, or replacing a part of a string of text. That's possible inline in any flow action. Microsoft Flow leverages the same Workflow Definition Language used by Azure Logic apps. For new users who are unfamiliar with the expression language, there is an inline help experience that shows how to use each expression as they build out their flow.
Box 4: flow type
There are four different Flow types that you can choose from when starting out. You can create a scheduled flow, automated flow, instant flow, or a business process flow.
Reference:
https://sharepointmaven.com/3-major-components-microsoft-flow/
https://www.avepoint.com/blog/office-365/office-365-automation/
https://flow.microsoft.com/de-de/blog/use-expressions-in-actions/