They all mean nothing because the sketch won't compile.
You've got a ';' instead of a '{' after "loop()'.
The value 'alarm' is not declared.
You can't return the value 'a' from loop() because the 'void' in 'void loop()' means "This function does not return a value".
If those are fixed:
"What does serial.printIn mean?" It means send characters to the Serial Monitor (if it is open) to display '0' (the value of 'a') and go down to the beginning of the next line.
"What does a! mean?" It doesn't. 'a' is an identifier. '!=' is an operator meaning 'is not equal to'.
'What does return a mean?' If loop() was declared 'int loop()' instead of 'void loop()' it would mean that the value of 'a' would be returned as the value of the function.