debug.log is used for log output, which is convenient for doing some simple debugging (such as the following example). Of course, for more precise and complex debugging, we must use mono's debug or vs's debug. .

int a = 10;

int b = 20;

int result = 0;

start(){

result = a + b;

debug.log(result);

}

This can be seen on the unity console The result of result: 30