Similarity is the share of your text that matches sources Turnitin knows: websites, publications and other students' papers. A match isn't plagiarism on its own; quotes and references match too, which is why you can exclude them.
AI writing is Turnitin's estimate of how much of the prose looks AI-generated. Scores from 1% to 19% show as an asterisk, because Turnitin found more false positives at that level. Like every detector, it can be wrong, so treat it as a signal, not proof.
If the AI score is higher than you expect, rewrite the flagged parts in your own words, or run them through the Unbot AI humanizer and check again.