Newbucks are the currency of Slime Rancher. Everything that is purchased within the game is paid for in newbucks. They can be obtained by selling Plorts at the Plort Market, opening Crates, or feeding Lucky Slimes.
By selling plorts they will decrease in price the next day. Make sure to stock up and don't use too many at once.
Newbucks are used to purchase structures and expansions for your ranch, and also to upgrade the Vacpack. They also can be used to upgrade pre-existing structures and purchase blueprints in the lab where the rancher does Slime Science, and to gain higher ranks in the 7Zee Rewards Club.