Debug mode for mobile apps mean that:

Assertions are enabled.

Service extensions are enabled.

Compilation is optimized for fast development and run cycles
(but not for execution speed, binary size, or deployment).

Debugging is enabled, and tools supporting source level debugging
(such as DevTools) can connect to the process.

Debug mode for a web app means that:

The build is not minified and tree shaking has not been
performed.

The app is compiled with the dartdevc compiler for
easier debugging.

By default, flutter run compiles to debug mode.
Your IDE supports this mode. Android Studio,
for example, provides a Run > Debug… menu option,
as well as a green bug icon overlayed with a small triangle
on the project page.

Note:

Hot reload works only in debug mode.

The emulator and simulator execute only in debug mode.