Underneath ProjectBuilder and Xcode’s build system is a bunch of shell commands. The BuildLog shows you exactly what commands are being run and what their output is. Certain errors, like undefined symbols, are not properly explained in the GUI view but are fully visible in the log.
The BuildLog is part of the Build window. It’s well hidden by a separator that is at the very bottom of the window by default. Drag the separator upwards and the BuildLog will appear.