know what it is to be (something)
To have thorough knowledge about or a lot of experience of being something. Don't stand there and act like you know what it is to be a soldier. You've never even been in combat. You should talk to Deborah. She certainly knows what it is to be a single mother running her own business.
know what it is to be/do something
have personal experience of being/doing something: I know what it is to be a mother, so believe me when I say it is very hard work.