I'll try. I'll start with the easy answer than explain why.
To be simple, think of those numbers as starting points before a game starts. So take the Bucs/Jets game where the line (the line is the term used) is -3 for the Bucs. So essentially, the Bucs are starting the game down 3-0 or if you look at the other side, the Jets are +3 meaning they start the game up 3-0.
That's a simple way to think in terms of the + or - numbers. The goal is to surpass whatever the number is with the team you pick. So if you pick the Bucs -3 then you are saying that they will win by 4 or more points. Conversely, if you pick the Jets you think they are going to win outright or at the bare minimum lose by no more than 2 points. If the final score is something like the Bucs winning 24-21, 17-14, etc. then it's exactly a 3 point difference and no one wins which is called a push and everyone gets their money back minus the vig, which is kind of like a fee to place the bet.
What handicappers and Vegas do is try to set the line at a reasonable end result to get bettors to bet on both sides because casinos have a goal of simply keeping the vig and giving a losers bet to a winning bet and have it be a wash. That's why if you're in a casino you can see some of the lines change like a stock market ticker. When that happens that means one team is being bet heavily and Vegas is trying to balance out the action.
The last part isn't important for a casual bettor, but is for the pros. Hope the first half helps. I think you've asked a few times over the years so maybe my explanation isn't the best.