It is the amount of SBD/Steem you get. If you have each post set to 50/50 (default) you will get 50% of each. SBD is Steem dollars which are basically a form of money (how many you get is calculated by that dollar amount). The other 50% you get as Steem which is power and affects basically how much money you can give others with your upvotes. The $ sign is how much each post is worth (in USD I believe).