What is free lay in sports betting

Many sport exchanges like betfair/betdaq where you can back and lay teams.Free lay opportunity always exist.
Free lay is situation where laying any team and chances of odd move upward more than downward.
Free lay also the odds which is layed and will stay for some time.
But the main thing is when free lay present and how we can assume it.
In this blog we will try to find it either inplay or before event.