线上 (xiàn shàng) is a noun meaning 'online', used as an attribute before a noun (e.g., 线上课程 'online course'). It contrasts with 网上 (wǎng shang), which is more colloquial and can also mean 'on the internet' as a location; 线上 is slightly more formal and often contrasts with 线下 (xiàn xià 'offline'). 在线 (zài xiàn) is a verb meaning 'to be online' (e.g., 我在线 'I am online').