Betting the spread means betting whether a team covers a margin, not just wins. A -6.5 favorite has to win by 7+; a +6.5 underdog can lose by up to 6 and still cover. Spread bets are typically priced near -110 on both sides.
Markets & bet types
Point spread
A handicap added to the underdog (or subtracted from the favorite) to even out the market.
Example
Lakers -4.5 vs Suns +4.5. If Lakers win by 5+, Lakers cover. Lakers win by 4 or lose outright, Suns cover.