A unit test checks one small part of a program on its own. It is like checking that one ingredient on a kitchen scale measures correctly before using it in a larger recipe.
For example, a unit test might give a price-calculation function two numbers and check that it returns the expected total. If the test fails, the problem is easier to locate than when only the whole app is tested.
Unit tests are quick and focused, but they cannot show that every part works together correctly. Larger checks are still useful for the full flow between screens or services.