Many grad programs at various universities and specialty schools will require you to write a personal statement in order for you to apply to the school and if you are not sure what to write then you need to know where to look so that you can figure it out. Because your personal statement will weigh heavily on whether or not you get accepted into the school of your dreams, you want to make sure that you write an admission essay that really wows those who will read it.
The personal statement is also a great place to explain more serious problems, like less_than_ideal grades or test scores. Don't be afraid to write something like, "I don't test well" in the statement. As long as you can provide a good reason for you app's problems, it's okay to address them directly. You won't be able to hide it from admissions officers anyway, so providing a reason for problems is always preferable to ignoring them.