TestVisible annotation allow test methods to access private or protected members of another class outside the test class. These members include methods, member variables, and inner classes.

Sample Class:

public class TestVisibleExample {
    // Private member variable
    @TestVisible private static Integer recordNumber = 1;


    // Private method
    @TestVisible private static void updateRec() {
    }
}  

Test Class:

@isTest
private class TestVisibleExampleTest {
    @isTest static void test1() {
        // Accessing private variable annotated with TestVisible
        Integer i = TestVisibleExample.recordNumber;
        System.assertEquals(1, i);


        // Accessing private method annotated with TestVisible
        TestVisibleExample.updateRecord();
    }
}