Originally literal —
当 (to be at) +
道 (the road) — meaning 'blocking the path'. Idiomatically extended to 'holding power' since at least the Han dynasty:
奸臣当道 'treacherous ministers in power' is a stock phrase in dynastic history. Modern usage: X
当道 = 'X dominates' (
短视频当道,
流量当道).