The Project menu on the menubar defines the standard Eclipse project build commands, including "Build Automatically". (See the figure below for the location of the menu entry.) Build Automatically is actually an option, not a command, and will have a check-mark next to it when it is enabled.
When the Build Automatically option is enabled and an Ada source file is changed and saved (or after the overall project is cleaned), the compiler is invoked. However, a full build is not performed. In this mode the compiler will perform syntax and semantic analysis on the unit in the changed source file and all of its dependent units, transitively. The more expensive binder or linker phases are not performed. Errors are thus detected as early as possible but without the expense of a full build.
It follows that a manually-invoked full build is still necessary to create an up-to-date executable. Explicitly invoking the builder will build that executable, as usual.