You Just never know when a pokemon might appear. Come here to find wild pokemon to catch or battle. Trainers may be lurking in here as well.
What to do?
When you enter the tall grass make a post saying you have entered the tall grass and what you are doing there.
Example
*soandso enters the tall grass. Is looking for a Pidgey*
This is just a very basic post. Remeber you control your character and anything is possible. You may say what pokemon you are looking for or maybe you just want a random pokemon to appear.