Bulkifying Apex Trigger 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 SOAP API call that inserted a batch of records. So if a batch of records invokes the same Apex trigger, all of those records need to be processed as a bulk, in order to write scalable code and avoid hitting governor limits.

Here is a simple example of some Apex code both bulkified, and not bulkified.

Not Bulkified:

trigger AccountTriggr on Account (before update) {
    
    //This only handles the first record in the Trigger.new collection
    //But if more than one Account initiated this trigger, those additional records
    //will not be processed
    Account acc = Trigger.new[0];
    List<Contact> conList = [SELECT Id, FirstName, LastName, Email, Phone FROM Contact WHERE AccountId = :acc.Id];
}

In above apex trigger 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 Trigger.new[0]. Instead, the trigger should properly handle the entire collection of Accounts in the Trigger.new collection.

Bulkified:

trigger AccountTriggr on Account (before update) {
    
    //Loop through all records in the Trigger.new collection
    for(Account a: Trigger.new){
        //Write your logic
    }
}

In above bulkified apex trigger the code iterates across the entire Trigger.new collection with a for loop. Now if this trigger is invoked with a single Account or up to 200 Accounts, all records are properly processed.