When betting on basketball, you can choose to bet on the number of assists that a particular player will have in a game.
With "players assists milestones" betting, you can bet on whether the player will hit an assist "milestone" in the game. For instance, Chris Paul's standard assists over/under line might be set at 10.5 assists.
His assists milestones lines, however, might look like this:
3 Assists, -1050
5 Assists, -350
10 Assists, -110
15 Assists, +375
20 Assists, +1000
As you can see, you can bet on Chris Paul hitting various milestones in the game.
Will he hit 3 assists? Very, very likely, which is why the line is set at 3 Assists.
Will he hit 15 assists? Probably not, which is why the line is set at +375, which translates into being a big underdog.