在线 is a verb meaning 'to be online', focusing on the state of being connected to the internet. It differs from 网上 (wǎng shang, 'on the internet') which is a noun used as an adverb for location (e.g., 网上购物 'shop online'), and from 线上 (xiàn shàng, 'online') which is more formal and often used in business or event contexts (e.g., 线上会议 'online meeting').