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]);
}

1. @isTest — marks the test method.
2. static void bulkInsertStaysWithinLimits() { — the test.
3. List<Account> accounts = new List<Account>(); — the data setup begins.
4. for (Integer i = 0; i < 200; i++) { — building 200 records.
5. accounts.add(new Account(Name = 'Test Account ' + i)); — everything here doesn't count toward the governor limits Module 25 covered.
6. } — closes the loop.
7. (blank line)
8. Test.startTest(); — Test.startTest() marks the boundary.
9. insert accounts; — governor limits reset here — this DML gets a fresh, separate budget.
10. Test.stopTest(); — marks the end.
11. (blank line)
12. System.assertEquals(200, [SELECT COUNT() FROM Account]); — confirms all 200 were saved.
13. } — closes the test method.

This lets 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());

1. Test.startTest(); — marks the boundary.
2. someService.queueAsyncWork(); — enqueues a Queueable job (a later module covers these).
3. Test.stopTest(); — Test.stopTest() has a second important effect: any asynchronous Apex enqueued since startTest() is forced to run synchronously, to completion, before this line returns.
4. (blank line)
5. // by this point, the async job has actually finished running — spells out that guarantee.
6. List<Result__c> results = [SELECT Id FROM Result__c]; — safe to check the async job's output now.
7. System.assertEquals(1, results.size()); — without this guarantee, a test asserting on the async job's results might run before that job had actually finished, in a real (non-test) execution.