The line is like a handicap the better fancied team give the lesser team and by doing so, even up the odds a little.
I hope you follow the footy
For example, say the Melbourne Storm on average will beat West Tiger most of the time, so on a head to head bet, the Storm is gonna be at a very short odds of say $1.15 and WT at $4.95
The betting agency then created this handicapped system called line,so in this case the Melbourne line will be like -11.5 and WT at +11.5 and the odds will be $1.95 a piece. A bit of level playing field.
So if the final score was Storm 20 and West Tiger 10, if you would have taken the line of WT +11.5 then WT's score will become 10+11.5 = 21.5 and you won!
Better now
Cheers