Some people have authority to criticize something because of their experiences. For example, if you work really hard every day, it's OK for you to criticize someone who doesn't work hard.

When someone doesn't have authority to criticize something, you can ask "Who are you to talk?" So, for example, if your messy roommate complains that you don't clean enough, you can say:

Who are you to talk? You're just as messy as me!

People also use this phrase to talk about themselves, to admit when they've said something that they don't have authority.

He's so disorganized. But who am I to talk? I can't even keep track of what day of the week it is.