A service layer groups application behavior into cohesive operations. It coordinates domain rules and dependencies while presenting stable interfaces to controllers, user interfaces, agents or other callers.
This separation can make code easier to test and change because callers do not need to know every database or transport detail. An overly broad service layer can still become a monolith, so responsibilities should remain focused.
ELI5
A service layer is a middle part of an application that knows how important jobs should be carried out. Other parts ask it for an outcome instead of repeating all the steps themselves.
For example, a checkout screen can ask an order service to place an order. The service coordinates validation and storage while the screen focuses on showing information to the user.
