deftest is a macro for defining a unit test, similar to unit tests in other languages.

You can create a test as follows:

(deftest add-nums
  (is (= 2 (+ 1 1)))
  (is (= 3 (+ 1 2))))

Here we are defining a test called add-nums, which tests the + function. The test has two assertions.

You can then run the test like this in your current namespace:

(run-tests)

Or you can just run the tests for the namespace the test is in:

(run-tests 'your-ns)