na-adjective
Describes something as typical of ordinary people, lacking refinement or aristocratic quality. Often carries a slightly negative or dismissive tone.
彼の趣味は平民的で、高級なものには興味がない。
His tastes are plebeian; he has no interest in high-class things.
そのレストランは平民的な雰囲気で、誰でも気軽に入れる。
That restaurant has a common, down-to-earth atmosphere where anyone can feel comfortable.
平民 is the noun 'commoner' or 'ordinary person'; 平民的 is the adjectival form describing something as characteristic of commoners.
Formed from 平民 (commoner, ordinary person) + the adjectival suffix 的 (-like, -ish). The base word 平民 is a Sino-Japanese compound of 平 (flat, ordinary) and 民 (people).