It means higher than the typical -110. The sports book knows it is giving you a line that you can't get elsewhere at that point in time, so they make you pay for it a little bit. The Jets right now are +10 with -120 juice. That means you wager 12/10 instead of standard 11/10.
However, if you want the Dolphins and think it will be well beyond 10, then it can be enticing because you get Even money (10/10) instead of giving 11/10.
Here is a chart from Vegas Insiders. You'll see what I mean if you look at the juice alongside the spread. Some might be -115 and not -120. That is only 5 cents more, not 10 cents more, in other words bet 115 to win 100.
Scroll down. Most recent number is lower on the list. You can see the time when the line changed:
Line Movements for Jets vs Dolphins provided by VegasInsider.com, along with more nfl information for your sports gaming and betting needs.
www.vegasinsider.com