That is the amount of starter wort that you pitch the yeast into.
If it is a small amount, say 1 liter or less you can pitch that whole thing at high krausen into your wort.
If it is more than 1 liter I chill it then decant most of the liquid. I leave a little to swirl up the yeast cake. This will be about 1/4 to 1/2 inch on the bottom of a 2L flask. A few milliliters. I don't know how many ml, I have never measured.