Hi,

This means you have "Debug" enabled on the tool bar.

When debug starts it sends a "hello" to the pc. In this case either the arduino or the pc has corrupted the message. 

In its default state the debugger shares Serial with your sketch code. Therefore it is encountrs any data it doesn't understand it outputs it to the serial monitor because it could quite easily be the result of your own Serial.print() statement. but in this case of course it is not your code printing the message.

You can ignore it or look into why the board is having a problem sending a serial message or switch the toolbar to "Release" and re-upload.

Some arduinos do have problems sending the first serial message. which is really the reason for the "hello". You should find debugger breakpoints work okay.