In bounded picklist user can't enter text in picklist field.

Where user can pick the data from the provided list is 
bounded picklist.

A bounded picklist is a picklist where users must choose 
from the existing choices and cannot enter their own data.
Whereas with Unbounded, u can enter you own values which is 
set to the field at the RunTime.

In siebel tools, in the object explorer, select PickList 
Property. To the PickList there is one Property 
called 'Bounded' set it to TRUE to make the picklist 
BOUNDED.

While you create a Picklist Bounded is property which you 
can have only True or False, once you check the property to 
True then you cant enter your date instead of You should 
pick the available values from the List of Values availabel 
in the Picklist Field.