Bulkifying Apex code refers to the concept of making sure the code properly handles more than one record at a time. When a batch of records initiates Apex, a single instance of that Apex code is executed, but it needs to handle all of the records in that given batch. For example, a trigger could be invoked by an Force.com Web Services API call that inserted a batch of records. So if a batch of records invokes the the same Apex code, all of those records need to be processed as a bulk, in order to write scalable code and avoid hitting governor limits.

Here is an example of poorly written code that only handles one record:

    trigger accountTestTrggr on Account (before insert, before update) {
 
       //This only handles the first record in the Trigger.new collection
       //But if more than 1 Account initiated this trigger, those additional records
       //will not be processed
       Account acct = Trigger.new[0];
       List<Contact> contacts = [select id, salutation, firstname, lastname, email
                  from Contact where accountId = :acct.Id];
       
    }

The issue is that only one Account record is handled because the code explicitly accesses only the first record in the Trigger.new collection by using the syntax Trigger.new[0]. Instead, the trigger should properly handle the entire collection of Accounts in the Trigger.new collection.

Here is a sample of how to handle all incoming records:

    trigger accountTestTrggr on Account (before insert, before update) {
     
       List<String> accountNames = new List<String>{};
     
       //Loop through all records in the Trigger.new collection
       for(Account a: Trigger.new){
          //Concatenate the Name and billingState into the Description field
          a.Description = a.Name + ':' + a.BillingState
       }
       
    }

For more examples, go through the URL http://sfdc.arrowpointe.com/2008/09/13/bulkifying-a-trigger-an-example/

Hope this helps.