Profiles are a very important and powerful security feature. The same way each user in Salesforce can have a role, each user must have a profile. Profiles allow you to set access to objects that are more powerful and overwrite other security settings. While profiles cover a multitude of settings, we will only focus on the object settings in the context of sharing and visibility here.
You are the Salesforce admin for XYZ Widgets. The sales manager has requested that a group of users, all of which have the Service Manager profile, should have View All access to the accounts object. You will do this by updating the Service Manager profile.