Depending on its position in a sentence or cluster thereof, a clarifying statement, a conversational stall, or a slightly apologetic amplifier. Rarely used in formal written communication, but common as a conversational particle in early 21st-century American English.
1. "That movie sucks
! I mean, the FX
were okay but the story was shit
Here, "I mean" is used to introduce a following clause which modifies the prior one. This usually has the effect of altering a very strong and general statement to instead reflect a specific contention.
2. "What's your favorite movie?"
"Hmm... I mean... I like Mission Impossible
, but I also love Die Hard
Here "I mean" covers the speaker's thought process and indicates that conflicting ideas are being weighed in the speaker's mind. It indicates that an answer has not yet been formulated, but does not invite the other speaker to continue talking.
3. "Man, that song is really great!"
"I mean, if you hate music."
Here "I mean" is used to indicate that the speaker has briefly thought of a more polite way to indicate the statement but can only express it in a blunt or sarcastic fashion. "I mean" only starts a statement of this nature if it is in response to someone else's statement, though it can be used in both to agree and disagree with said statement.