na-adjective
Describes situations, events, clothing, or language that is casual and not official. Often used in contrast to フォーマル.
今日のパーティーはインフォーマルな服装で大丈夫です。
Casual clothes are fine for today's party.
インフォーマルな会合なので、敬語
It's an informal gathering, so you don't need to use keigo.