Others have explained it more succinctly in the past, but it's kinda like a near-even-odds bet on either side of the predicted margin. When there's one figure, it's listed for the first/home team. Think of it like a handicap/adjustment for an evenish bet (e.g., $1.90).
36ers 2.75
Cats 1.45
Line +4.5
So, this says that they think the Cats will win by 4-5 points. The half-point gives you a clear margin - basically a two-horse race. They think that Adelaide would need a +4.5 headstart for scores to finish level, if that makes any sense.
If it were -9.5, they're expecting Adelaide to win by 9-10ish. There will be a line and then other lines you can also bet on for longer or shorter odds.
e.g., when I said that Adelaide were paying $8.30 for a double figure win, I meant Centrebet were offering $8.30 for a line of -9.5. For a line of +19.5, they might only offer $1.05 - you'd collect on that if Adelaide either won, or lost by less than 20 points (e.g., not so common in an even competition).
For a practical example, if you thought the Cats would win, but didn't like the $1.45 odds, you might figure "Well, if they win, given injuries, it'll probably be by 6-7 points," and take $1.90 instead. It just adjusts the result you're betting on a little. Instead of win-loss, you're betting as though the definition of win or loss was tweaked.
One bit of advice I read once suggested avoiding the "exotics" - things like first to score, player to hit most threes, opening score type, top scorer, etc. Instead, go for the line or under/over (total game score).
Unless he only ever tells me about his successes and there are loads of failures, I believe DJ on here has some luck with the under/over bets.
In that case, they will say something like 192.5 and you will be betting whether you think the combined points total of both teams will be under or over that. e.g., Blaze-Breakers might be more inclined to be high scoring, while Cats and past Taipans teams were usually a grind. The bookies don't have a blanket figure though -- they will adjust this depending on the match-up, past stats, etc.
As always, you're banking on your knowledge of the league (or pure luck) topping that of the bookies.