If you happen to collect all 900 korok seeds, you will be rewarded with an Hestu’s Gift, a gift of friendship given to you by Hetsu. It smeels pretty bad. Here is the definition of Korok, it is a small seed given by a Korok, it has a distinct smell. If you gather a buch of them, you never know what may happen.
Basically, you can find this cute korok anywhere hiding from rocks, blocks, boulder hole, flowers, ice or even fruit trees. The challenge is completing all the 900 seeds is a little bit hard as you will roam all the map to find this korok.
To help you, someone uploaded a full map in Reddit by a user BigMoney_D and all of the 900 korok seeds are plotted in the maps for your guides. See the maps below.
Enjoy collecting the korok seeds!