Validation Rule Example

Validation rules verifies that the data a user enters in a record meets the criteria that you specify, before the the record is saved.

example- 

If FieldA  is selected to any of these

High School,

Middle School,

Elementry School

then 

FieldB has to be true. 

AND(

OR(

TEXT(FieldA__c) = 'High School',

TEXT(FieldA__c) = 'Middle School', 

TEXT(FieldA__c) = 'Elementry School'

),

TEXT(FieldB__c) = false

)

Another simple Example of validation rule - https://www.youtube.com/watch?v=IkXqE--1QsE