A couple of my buddies are confuse to what the following means

$\displaystyle EF$ $\displaystyle :$ $\displaystyle FO$ $\displaystyle ::$ $\displaystyle DB$ $\displaystyle : $ $\displaystyle BO$

We are looking at length of line segments. I was telling them that the colons mean that they are proportional.

What do the colons mean?