na-adjective
see-through; sheer; transparent
Describes clothing, fabric, or materials that are thin enough to see through. Often used in casual conversation.
このブラウス、すけすけだから下にキャミソールを着たほうがいいよ。
This blouse is see-through, so you'd better wear a camisole underneath.
すけすけのカーテンだと外から丸見え
With sheer curtains, you can be seen clearly from outside.
透明 is a more formal and neutral word for 'transparent', while すけすけ is casual and often implies something is revealing or too thin.
Reduplication of 透ける (すける, 'to be transparent'), emphasizing the see-through quality.