Take Lightly Meaning

(idiomatic, transitive) To approach a task casually.

Example: 2011 February 12, Mark Orlovac, “England 59-13 Italy”[1], BBC:
  Following England's opening-day win over Wales in Cardiff, Martin Johnson's men headed into the Twickenham contest full of confidence but with a warning from their manager not to take the Italians lightly.