Basically the Chiefs -7.5 (this is the spread) means betting on them is saying they'd win by more than that amount (they'd 'cover' the spread). Otherwise you could bet on the Titans, they are the underdog who are being 'given' 7.5 points, if they win the game, or lose by less than 7, your bet pays out.
This number changes based on the money coming in and other circumstances (like injuries during the week), but mostly that first bit, to spread exposure for the sportsbook.
Chiefs were -9.5 iirc against the Texans, so anyone who bet on that game were down over 30 at one point from cashing those tickets, even more incredible comeback from a gambling perspective.
But what do I know, I'll be 41 in a couple weeks, I'm a Las Vegas local and I've never placed a bet.
I haven't done fantasy football in years, but I really enjoyed playing in pick'em leagues w/ the betting lines on. It's easy to pick favorites all the time, but picking a good team to win by 14.5 or more? That's a more fun pick'em to me.