Hi Punta
The line is the points start one team gets to make a bet relatively even. In the following example from Centrebet, the line on Melbourne is -3.5, ie they must win by 4 or more to win. Tennis is more hectic on Monday then any other day and is more active on Monday to Thursday then the weekend. I agree the prices on favs in tennis is horrible.

Win Line

Melbourne Storm $1.67 -3.5 $1.92
North Queensland $2.15 +3.5 $1.88