I thought I could outsmart craps one time and lost $300. That was the only and last time I gambled.

When a team is +number, that means that they are favored to lose by that amount.

When a team is -number, that means that they are favored to win by that amount.

Almost always the number will be 1/2 a number (i.e +7.5, -3.5, +17.5....etc) so that there aren't any "ties". Either vegas wins or you win. If you bet that your pick will "cover the spread", you're beating that they'll at least win or lose the amount they were predicted to win or lose.

Hopefully that helps