The process should be familiar if you played last season
Depending on the season, Fortnite might add a core mechanic to the game that’s either gone forever once a season is done: or added permanently/for an extended period of time. Reality Tree Saplings have been incorporated in the universe for a good part of this year, and the challenges associated with them are fairly simple to clear.
How to clear the Fortnite “Pick fruit from Reality Saplings” challenge
To pick fruit, you’ll need to plant seeds first.
To do that, you’ll locate a reality pod (a blue plant-like bulb) on the map. These can be found all over, but a few of the most popular spots to look right now are Chromejam Junction, as well as above Shiny Sound. They’ll look like blue pods with little branches twisting out of the ground. You can shoot the blue pods to release the saplings therein: but it’s easier (and faster) to just climb up and hit it with your pickaxe.
Once it blows up, sapling seeds will be strewn about the map: pick those up and throw one on the ground to plant your own sapling (or use them in the same, or subsequent games to recall it to your location). Note that placement matters, and it might not “take” if you place/throw it in an area without solid ground. Take notice of the blue holographic-like overlay when figuring out spots to toss them.
From here, you just need to be patient and let your sapling grow and yield fruit. If there are weeds around your sapling: clear them out (just interact with them directly like you’re picking up produce off the ground in the wild). Over time you’ll be able to pick the fruit off of the sapling, and the quest will be finished.