Used to express mild doubt or hesitation about an idea without directly disagreeing.
“I'm not so sure - that restaurant gets really busy on weekends.”
“I'm not so sure about taking the highway; there's usually construction there.”
This softer disagreement phrase often appears in Listening Part 1 when one speaker questions a suggestion the other has just made. Notice it because it usually leads into an alternative idea or a concern, which may be tested in comprehension questions. Don't confuse it with 'I'm sure' - the 'not' completely changes the meaning.
Synonyms: I have doubts, I'm hesitant, I'm uncertain about that