HA! that's a great question, and the answer is NOTHING: because the sample tests you showed are from some other kata, but the idea is the same. Anyway, the sample tests for this kata are:

Test.describe("Basic tests")
Test.assert_equals(bmi(50, 1.80), "Underweight")
Test.assert_equals(bmi(80, 1.80), "Normal")
Test.assert_equals(bmi(90, 1.80), "Overweight")
Test.assert_equals(bmi(110, 1.80), "Obese")
Test.assert_equals(bmi(50, 1.50), "Normal")

...so your function bmi() is passed two parameters and you should return a str as a result.

• test.assert_equals () makes a comparison of the two items passed into it
• [- 1, -2, -3], [-4, -5, -6] is the given input for this test
• array_plus_array() is the function you need to write
• 21 is the expected return value for this test
• so the tester compares the value your function returns to the expected value