They're the handles of the line objects it created. They've been cast to double (because it's rather old code), but if you're using R2014b or newer, you can get to the objects like this:
load carsmall
h = boxplot(MPG, Origin);
h = handle(h)
h =
7x6 Line array:
Line Line Line Line Line Line
Line Line Line Line Line Line
Line Line Line Line Line Line
Line Line Line Line Line Line
Line Line Line Line Line Line
Line Line Line Line Line Line
Line Line Line Line Line Line
Then you can set properties on them, such as color and line width.
