A current of water below the surface and moving in a different direction from any surface current.
An implicit quality, emotion, or influence underlying the superficial aspects of something and leaving a particular impression.
Genius songwriting on Steve Knightley's part, as the word Undertow serves to both show the underlying emotions of characters in the song wanting to leave England, while also "staying" in the Undertow, in which they'll continue to be dragged in a different direction than where their dreams should be leading them.
Definition for Undertow: