1. Build the project.
a. Select the build configuration. (optional)
b. Click on Build.
c. Check there are no compiler errors.
2. Configure the debug configuration to start a debug session.
a. Click down arrow next to Debug button.
b. Select 'Debug Configurations…'
c. Select the debug configuration associated with your current build configuration.
d. Select whether to rebuild code each time debug session start is requested.
e. Click on Debugger tab.
f. Verify proper interface and port.
3. Click Debug
What's the difference between Debug mode and Release mode?
The difference is mainly in the compiler switches. Debug mode produces a build containing the debug information required for debugging, but it is much bigger code size. The Release mode is more code-size efficient, but you cannot debug. So Debug mode for development activities, Release mode for building your final product.
You can find a well-written and fully detailed explanation by our colleage Erich Styger: Debug vs. Release? | MCU on Eclipse
Hope that helps!
It's useful and clear!