In betting exchange markets, there are commonly used expressions like BACK and LAY.
So what’s all about ?

BACK is : When you WANT this to happen

For example
Chelsea vs West Brom, BACK CHELSEA
RESULTS
Chelsea wins : You win the bet
Match ends draw : You lose the bet
West Brom wins : You lose the bet

Another example
Man Utd vs Arsenal, BACK DRAW
RESULTS
Man Utd wins : You lose the bet
Match ends draw : You win the bet
Arsenal wins : You lose the bet

On the other hand, we see the LAY
LAY is : You DON’T WANT this to happen (you bet against it) TO WIN

For example
Chelsea vs West Brom, LAY WEST BROM WIN
RESULTS
Chelsea wins : You win the bet (since West Brom DID NOT WIN)
Match ends draw : You win the bet (again since West Brom DID NOT WIN)
If West Brom WINS : You LOSE the bet

Same situation as LAY CORRECT SCORE
Chelsea vs West Brom, LAY 0-1 CORRECT SCORE
RESULTS
You will LOSE ONLY if the match ends 0-1
You will WIN in ANY OTHER SCORE RESULT (example 1-0, 1-1, 2-2, 3-1, 2-0, etc…even 0-2)