REWRITE: People sometimes say, "So do I," but I don't understand what it means. Could someone explain to me what it means? Could you give me an example, please!
"So do I" is a quick way to agree with someone.
A: I love snowboarding. (affirmative)
B: So do I. I do too. Me too. I love snowboarding too.
A: I DON'T like chocolate ice cream. (negative)
B. NEITHER do I. I DON'T either. Me NEITHER. I DON'T like chocolate ice cream either.
"Me too" and "Me neither" are very informal.