An I-statement is exactly what it says on the tin: a statement that begins with the word "I". Phrasing things in this way can be used to soften a confrontation. For example:
"I think you watch too much TV."
Is a bit kinder than just:
"You watch too much TV."
The latter is a "you-statement", which places the emphasis on something the second person has done wrong. With an I-statement, the statement emphasizes a perceived wrongdoing, which can be an important social distinction. (Consider also the necessity of saying "in my opinion" after controversial statements.)