Sorry for the late reply LG.

Line betting is like handicap betting. Basically it provides a scoring point that the bookmaker believes handicaps the match into a 50/50 result. Two examples:

AFL - West Coast Eagles vs Sydney Swans
The line might be -19.5 to west coast which means its +19.5 for sydney. All this essentially means is at the end of the game which ever team you bet on you subtract (west coast) or add (sydney) 19.5 to their score to see if they then win. So say the result was 102 (WC) to 95 (SY), then if you bet on west coast to cover the line you'd take 19.5 from their score which then becomes 82.5 (WC) vs 95 (SY) so you would lose your bet. Conversely if you'd bet on Sydney at the line you'd add 19.5 to sydney's score to see if they won which becomes 102 (WC) vs 114.5 (SY) and you'd have won your line bet.

NBA - Cleveland Cavaliers vs Indiana Pacers
Same as above say the line is -6.5 to Cleveland and +6.5 for Indiana. This just essentially means that Cleveland have to win by 7 or more if you bet on Cleveland at the line and Indiana cannot lose by 7 or more if you choose to bet on them at the line. So Indiana could still lose by 6 points or less and you'd still win your line bet if you bet on Indiana.

Fire away if you have questions I hope the above explains it well enough, it can be a hard concept to grasp initially but quite easy once you get the hang of it. Just need to think of it as a handicapping system basically.