Test.startTest and Test.stopTest

@isTest
static void bulkInsertStaysWithinLimits() {
    List<Account> accounts = new List<Account>();
    for (Integer i = 0; i < 200; i++) {
        accounts.add(new Account(Name = 'Test Account ' + i));
    }

    Test.startTest();
    insert accounts; // governor limits reset here — this DML gets a fresh budget
    Test.stopTest();

    System.assertEquals(200, [SELECT COUNT() FROM Account]);
}

Everything before Test.startTest() (building 200 Account objects, here) doesn't count toward the governor limits Module 25 covered; everything between startTest() and stopTest() gets its own fresh, separate limit budget — letting a test set up substantial data without that setup competing for the same limits as the actual code being verified.

Test.startTest();
someService.queueAsyncWork(); // enqueues a Queueable job (a later module covers these)
Test.stopTest();

// by this point, the async job has actually finished running
List<Result__c> results = [SELECT Id FROM Result__c];
System.assertEquals(1, results.size());

Test.stopTest() has a second important effect: any asynchronous Apex enqueued between startTest() and stopTest() is forced to run synchronously, to completion, before stopTest() returns — without this, a test asserting on the async job's results might run before that job had actually finished, in a real (non-test) execution.