Even though the above statement sounds simple, a manual tester involves himself/ herself in a number of different activities. Some of the prominent ones being:
Generating test documents: While testing an application, a tester has to firmly adhere to a plan and document the items that need testing and keep track of the items that have already been tested. So a manual tester creates test strategies, Test scenarios, RTM, test cases etc. These documents act as a concrete base for all the future manual testing activities performed by the quality assurance team.
Test case execution: The test cases once devised are executed and the status of each and every test case is conserved
Error reporting: If any test case malfunctions, a manual tester has to log the error with appropriate steps and other information so that developers or other testers can reproduce the defect conveniently.
Regression testing and retesting: Once any bug is corrected, the testing team retest the issues to make sure that they have been rightly fixed. Regression testing is performed to make sure that these bug fixes do not impact any other feature or the software flow.
Apart from these affairs, a manual tester should also take part in requirement grooming calls and release planning to understand the features being developed and to give estimates for the various testing tasks.