This nectar comes from the giant lily pads that are on the river of the Fungal Plains within the Mushroom Island.
Info in Slimepedia
This jar of syrup-like nectar is acquired from the bees that reside within hives under the ground of the Fungal Plains. If you take it out of the jar that it's in and play with it, it bounces in a glob pretty far, like a few inches in the air if you drop it against a surface. It rebounds pretty far if you lob a handfull off a wall, so I guess that you can play a game with your friends with bouncing it off a wall.
On the ranch
Deposit a jar of Bouncy Nectar into the refinery at the lab so it can be used as materials for various different gadgets, like Advanced Spring Pads that spring you 3 times higher than regular ones, or Boxing Gloves that bonk tarr into next year!
It's a yellow goo that has many small bubbles floating around aimlessly in a glass jar with a yellow lid. The nectar in the jar also has purple specks in it that seem to be spores from the surrounding fungal columns and mushrooms.