A negative test case is when the test case is designed to determine the response of the product outside of what is defined.
Consider a webpage containing a textbox whose specified requirement in the SRS document is to accept only numbers. In that case when we perform positive testing, we will give only numbers as input data in the textbox and analyse whether the application is working properly or not. Here, our expectation is that the text box should accept the given input values.
But when we perform negative testing, we will give text or alphabets as input data in the textbox and ensure that application wont accept this entry and throw error.