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 builder is automatically invoked.
If the project is clean (or has never been built in the first place), Eclipse invokes a full build and an executable will be generated if possible.
However, a full build is not invoked when Eclipse does not consider it to be required. For example, after a full build, changing a single file invokes an incremental build. 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, but the binder or linker phases are not performed. Errors are thus detected as early as possible but without the expense of a full build.
Note that the Ada editor has a preference, enabled by default, to save altered project source files before a build begins. If multiple source files are altered but not yet saved and the Build Automatically option is enabled, saving any one of those files will invoke the builder and thus all of the altered source files in that project will be saved as well.