It means you reached a certain level from which you can customize the amount of voting power you put with each vote. This is a standard option in Steemit, everybody has it, once they get over a certain level of SP. It helps you manage your voting power during the day (you have only 100 votes that are carrying power during 24 hours).
Hope this helps :)
It helps! Was this recently implemented? At which point do you get this option? Thanks!
It's been there for a long time but you didn't see it because it only gets activated once you reach a certain level of SP.