Human driven testing activities, that are not using or focused on building automated checks. Manual testing may still make use of tools and code, but is driven by a human and not a computer.
A potentially broad term, manual testing could be applied to any part of software testing. However there is a good chance, people use the term manual testing to mean, human driven test execution.
The term can be controversial, because it is sometimes used to down play the skill needed for testing, outside of the value of automation.
It used to be said, that there is no such thing as manual testing, because all testing is manual in the same way that all development is manual. This line of thinking is currently being challenged by the introduction of generative Ai tools that are, with varying degrees of success, attempting to automate development, as well as testing.