Projects are used in Google Cloud Platform (GCP) to organize resources. Very similar to Resources Groups in Microsoft Azure.
Use Projects to assign permissions (IAM) and manage billing. There can be a project for service1 in the organization on which service1 team will have permissions. There can be a project for department1 in the company, so they are billed for the cost they incurred for their project.
Resources in the same lifecycle should be part of the same project. For e.g. There can be a project for Dev and anther project for prod. You can delete a project to remove all resources that are part of the project. Most resources are retained for 30 days before they are not recoverable.
Every project will have a Project Name, Project ID and Project Number. Provide a descriptive Project Name and ID, Project Number are generated by Google and are Unique across the GCP.
Before Projects we have Organization (available when you signup GCP using your G-suite account) and Folders. Its highly recommended to organize projects under organization and Folders that represent your Org Hierarchy. Below is an example picture from Google documentations.