The main meaning is "noticeable in a negative way". For instance, "The man had an obtrusive scar on his face." A person can also be obtrusive. It means that he/she imposes himself/herself too much on others. For instance, if you try to impose your opinion on others (= to force others to agree with you) you're being obtrusive. Literally, obtrusive means to protrude, to stick out, so you can say that a "rock formation" is obtrusive.