Bulkifying Apex code refers to the concept of making sure the code properly handles processing multiple records at one 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 a Force.com SOAP API call that inserted a batch of records. That trigger would need to be able to handle at least 200 records since triggers break up records into 200 record batches. As a rule of thumb make sure all apex code written can handle at least 200 records at a time. This number could be much greater as well for custom methods querying against large datasets.