It is an expression that means "my opinion." "my two cents" = "my opinion"

It depends on the tone of voice that the speaker is using when they say 'but that's just my two cents.' Sometimes they actually don't think that their opinion matters very much but they still want to be part of the conversation.

It's like your opinion is only worth 2 cents, which is not a lot. So basically you're telling someone your opinion on a topic or you're giving advice to someone and then you're adding that your opinion isn't worth much, yet you still told it anyway. It can come across as passive aggressive. Like "listen to my opinion even though you might not want to and I know that but still I'm going to say it anyway." Examples: Big cities are polluted and therefore I would never move there. But that's just my two cents. (Maybe this is said after someone told them they were considering moving to a big city.) I think you should not go to the concert because concerts are loud. But that's just my two cents.