Tall Grass
In the Tall Grass you can look for eggs that have been sent to the wild. You need to purchase an 
Egg Radar from the Item Shop in order to access to it.
How to access
You can access the Tall Grass if you have an Egg Radar by hovering over the Town Tab, located at the top of every page.
Egg Radar Chips
You need to have 
Egg Radar Chips in order to search for eggs in the Tall Grass. There are 6 different chips, one per egg rarity (Easy, Medium, Hard, Rare, Starter, Legend). These can be purchased from the Item Shop if you have the Egg Radar.
Finding Legendary Pokemon
The only Legendary Pokemon which can be found in the Tall Grass are Latios, Latias, Keldeo and Cosmog. These Pokemon can only be found if you have the correct items - including the Legendary Radar Chip.
Latios & Latias can be found if you have an Enigma Stone. Enigma Stones are found rarely from Mystery Boxes. Please keep in mind that it is completely random which legendary you will find.
Keldeo can be found if you have a Resolute Stone. The Resolute Stone can be obtained in the Lab from Prof. Rowan through an undiscovered method, and it can also be found in Mystery Boxes.
Cosmog can be found if you have a Nebula Stone. The Nebula Stone can be found through Rumbling, specifically the Alola area that has to be unlocked first.
Searching eggs
Choosing the Pokémon
Once you own the Egg Radar and at least one chip, you can search for eggs in the tall grass. To do so, choose a Pokémon from the dropdown menu. In the menu, the Pokémon are sorted by dex number. The number of that kind of eggs are in brackets.
Important note: You can only search for eggs that are recorded in your Eggdex and you have the chip corresponding to their rarity.
(For example: If you have Psyduck's eggdex entry and you have the Medium chip, you will be able to search for it)
Once that have selected the Pokémon, press "search" and the searching will begin.
Searching the eggs
Once you made your selection, the radar will start searching for the egg. Thanks to the recent update at the end of October 2017, the eggs no longer move around like they did before. In other words, it will stay in a fixed spot until you find it. Whenever you click on a grass spot, you will get a notification that helps you to determine if you are close to the Pokémon egg or not:
"But you are very close!" → the egg is within the range of a 3x3 field
"The egg should not be far away." → the egg is within the range of a 5x5 field
"The egg is very far away." → the egg is outside the range of a 5x5 field
(Note: remember that the grass spot you picked is always the center of the given range!)
With these hints given, you can easily come up with the best strategies to find an egg as fast as possible. Also, keep in mind that there is a cooldown between each try to find the egg.
Information about the time and bushes depending on egg rarity
| Egg rarity | Grass field size | Number of grass spots | Cooldown time |
|---|---|---|---|
| Easy | 3x3 | 9 | 3 minutes |
| Medium | 4x4 | 16 | 10 minutes |
| Hard | 5x5 | 25 | 20 minutes |
| Rare | 7x7 | 49 | 30 minutes |
| Starter | 8x8 | 64 | 1 hour |
| Legend | 12x12 | 144 | 1 hour |