Korok Seeds can be obtained in Chapter Quests and certain Challenge Quests by uncovering hidden Koroks, and can additionally be received as rewards for completing Challenge Quests centered around Hestu’s involvement. Once a Warrior locates a Korok, it will give them a Korok Seed. Koroks who Warriors find will remain out in the open as a way to help players keep track which Koroks they have found.
In exchange for Korok Seeds, Hestu will add one slot to Link’s Weapon, Bow, or Shield stashes.  Link can fully upgrade his inventory with only 441 Korok Seeds. If all 900 Seeds are returned to Hestu, he will reward Link with a “gift of friendship.”  
Korok Seeds are given by Koroks when they are found throughout Hyrule.  They possess a distinct smell.  When Link receives his first Seed, he is asked to return it to the Korok Hestu.  Korok Seeds are actually the beads used in Hestu’s Maracas. When first found, Hestu asks Link to retrieve his Maracas from a group of Bokoblins nearby. After Link returns them, Hestu realizes the Seeds were taken by the other Koroks as part of a prank.  
Breath of the Wild
There is a total of 175 Korok Seeds found in the game, and are divided as such:
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Location and Uses
Once Hestu is unlocked as a character after completing the Scenario, “Freeing Korok Forest”, Link will be able to complete side quests that will be able to give everyone extra inventory, just like in Breath of the Wild. These side quests, found under the Battle Support tab in the Quest search bar, have Hestu’s Maracas as their icons.
After completing the “A Most Peculiar Korok” Side Quest, Korok Seeds can be seen on the Battle Info screen of a Scenario that has them, with collected Korok Seeds being colored in white and uncollected Korok Seeds being left in a very dull dark gray.
There are 900 Korok Seeds scattered throughout Hyrule in Breath of the Wild, and players will receive a reward for bringing all of them to Hestu.
BOTW’s Korok Seeds Reward Is A “Kin No Unko”
In order to max out Link’s inventory capacity, only 441 need to be collected. A very sizable amount will be found through the exploration required to complete the game, but getting to 441 will require some searching. Getting all 900 Korok Seeds in Breath of the Wild is a whole other story, though, either requiring players to use a Korok Seed guide and map, or spend dozens of hours combing the landscape.
The only practical use for Hestu’s Gift in Breath of the Wild is to make the giant Korok perform his signature dance on command, but Hestu may have given it to Link as a good luck charm. According to the Japan Times, a small, golden pile of poop is known as a “kin no unko,” and is often seen as a token of good luck. The little golden trinkets became popular in the early 2000s, when they were bought as funny souvenirs designed to lighten the mood and bring luck amidst an economic depression, but the concept may be much older.