Not trying to look better, richer, cleverer or more important than you really are — and said with warmth. This is praise. Calling a person unpretentious means they have no airs: a famous professor who eats in the staff canteen and lets you call them by their first name. Calling a place unpretentious means it is simple and honest and does not pretend to be fancy — an unpretentious family restaurant, plastic chairs, excellent food, no white tablecloths. It also works on things people often show off with: unpretentious clothes, an unpretentious wine, an unpretentious style of writing. Notice what makes the praise possible: the word only really applies where showing off would have been easy or expected. You would not call a poor student unpretentious, because they have nothing to pretend with; you call a rich or successful or highly educated person unpretentious, and the compliment lies in the gap between what they could claim and what they do claim. English is short of positive words for this — humble can sound religious, modest can suggest shyness — so unpretentious fills a real gap, which is why food writers and reviewers lean on it so heavily. It is a moderately formal word, common in writing and in careful speech.