'I mean' can be used to mostly two scenarios.
1. To follow up on an explanation on what someone said.
Ex: Let's tax more on the rich. I mean, our country has a huge budget deficit.
Ex: This person was horrible. I mean, he was throwing things at me!
2. Used to correct what someone said.
Ex: I blame the students, I mean, the parents.
Ex: He speaks Spanish... I mean, Italian.