It says 個性 や自由 koseai ya jiyu "Individuality and freedom" and I think it says はみ出してく hamidashiteku "to stick out" on the back, but it isn't clear. It is part of their motto that they did at the start of Otona Blue The First Take. Though since about 2021 they have been wearing socks that say 青春日本代表 Seishun nihon daihyō "Youth representative from Japan". They occasionally wear socks with other messages such as "Moving away from intolerance" or "I get lost but move on, that's what youth is".