regarding "this" and "this.value"

I am at the beginning of designing a web page. I was trying to design a login page and searched for the same and found something but I am confused with a line

onBlur="if(this.value=='')this.value='Username'" onFocus="if(this.value=='Username')this.value='' ">

please tell me about onBlur and onFocus and I am confused with "this" and "this.value" and also the if statement. Please help me out. Thanks in advance.